But now old friends are acting strange They shake their heads, they say I've changed Well something's lost, but something's gained In living ev'ry day - Both Sides Now ©1991, Southwest Publishing Corp.

JONI MITCHELL wrote those words and set them to music more than a decade ago. Over the years since then, she's given her friends and admirers many reasons to shake their heads in bewilderment as she's moved through some vivid shadings of self-expression.

Her most recent project - a jazz collaboration with the late bassist, Charles Mingus - has been an acid test of loyalty to an artist whose earliest influences were folk and rock 'n' roll. All of those flavors will be displayed tomorrow night at Forest Hills Stadium, where Mitchell will perform with the Percussions and a band composed of five jazz musicians with solid credentials of their own: Don Alias, Michael Brecker, Pat Metheny, Lyle Mays and Jaco Pastorius.

Sales of the "Mingus" album seem to indicate that Mitchell has succeeded not only in carrying her longtime admirers along on her excursion into jazz, but has won attention and respect within the jazz community as well. According to her label, Elektra Asylum, "Mingus" has sold about 300,000 copies since it was released in mid-July.

"I didn't think it would do that well," Mitchell said over the phone Wednesday night after a concert in Baltimore. "I knew it was worthy of that kind of reception, but I didn't expect it. It's very good for a jazz album. Unique, almost - isn't it? It's very gratifying to me."

And to many others, including Mingus himself, who lived long enough to hear her sing four of the songs he'd written for her. Both of them regarded the collaboration as a success, although it had begun in a most obscure way.

"The idea came originally from an Italian film producer by the name of Daniele Senatore, who was a very close friend of ours," Mingus's widow, Susan Graham, recalled earlier this week. "When Charles took ill, I called a number of people, hoping to find those that might commission works for Charles to compose. It was always a stimulus to work on order, and it was one of the last things that he was able to do as his paralysis set in. So I called Daniele, and be in the course of the next couple of weeks came up with this idea of a collaboration with Joni Mitchell.

"Mingus liked and admired her work. Everybody in the beginning had misgivings. But the moment Joni delivered that first tape, it knocked him out. She did an almost a cappella version of 'A Chair in the Sky.' It was very moving, the lyrics were very beautiful, Joni sang it magnificently and Charles loved it. And that really got the project rolling."

The disease that would finally kill Mingus - amyotrophic lateral sclerosis, the same disease that brought down the Iron Man of baseball, Lou Gehrig - 38 years ago - robbed him of his dexterity so rapidly that he was forced to compose by singing melodies into a tape recorder. His friend Paul Jeffrey then transcribed them onto the piano under verbal instructions from Mingus and sent the tapes to Mitchell.

"It was a piano and metronome tape that I first got," she said, then laughed and added, "The metronome nearly drove me crazy. It was like the crocodile and Captain Hook in 'Peter Pan'."

Both Mingus and Mitchell have reputations within their respective spheres for complex, highly personal forms of expression. But the same keen sophistication that made their collaboration possible in the first place nearly became a roadblock as the project went on, as Mitchell remembered.

"Some of them were difficult themes. Some of them I started and I deviated slightly from the theme that Charlie suggested and even had his approval on it. The way I write songs is... I rely heavily on incidents that occur, and then I transcribe them. Some of these were very difficult. For instance, on one of them I said, 'What is this song about?' and he said, 'This is about Charlie Parker in a good suit and a good haircut.' And then he described how Charlie Parker, that great saxophone player, would come down into the cellars of the New York jazz clubs in a silk suit and a cashmere coat and a fancy haircut at a time when everybody had goatees and sandals and berets on. They didn't think he could play jazz because of his appearance. And then when he played thirty-second notes, they thought he was just a weirdo. So he wanted me to tackle that. Well, I felt handicapped having not been present during that time and seeing it all with my own eyes.

"I had the same difficulty initially with 'Pork Pie Hat.' Charlie assailed me with all kinds of detailed incidents from Lester Young's life (the song is a tribute to that jazz musician) and at first I thought, 'It's presumptuous of me just to be a historian.' But then some things went down in the present tense that I experienced firsthand that were very related to Lester and Charles. I stumbled upon a bar - the Pork Pie Hat Bar. It's in the W. 50s. We got off the subway at the wrong stop and we saw a crowd gathered about two blocks away. When we came upon it, there were these two little kids dancing, and with the crowd gathered it was colorful by its own right. But then we looked up and what was under the awning was the Pork Pie Hat Bar. And the next bar, the next neon sign, said 'Charlie's' in red neon. So we felt really like magic was afoot. Don Alias and I witnessed it and we were both very excited about it. So that song fell into place with little snatches of past-tense history and future and present, and that's what makes a good song to me."  

Mingus died in Cuernavaca, Mexico, on Jan. 5, having heard all of Joni's songs except one - "God Must Be a Boogie Man," which she adapted from the opening pages of his autobiography, "Beneath the Underdog." But according to his wife, Mitchell's "Mingus" should not be taken as a eulogy.  

"Charles never intended a eulogy," she said. "Charles worked very hard on whatever project he was working on, and this was the one he was working on then. He was trying very hard to get well. We went to Mexico to follow a number of potential cures down there. They didn't work, but certainly we were trying and absolutely not giving up and surrendering to this idea that there was no way out. Mingus was at all times hopeful. He never gave up. He had been delivered his death sentence, but Mingus was a fighter."

And she believes the influence of his music will grow as the years go by.

"He's left an enormous legacy of music to the world, to be used as people choose to use it. I believe very much that his music is going to be taken and interpreted in all kinds of ways. And Joni's has certainly been done with the greatest sort of devotion and dedication that anybody could possibly give. She invested a year of love and time and effort in this project, and it was a source of comfort and interest and excitement to Charles to have this project going on in this period that was generally pretty miserable."

Mitchell shies away from attempts to put a label on what might come next. "This is jazz because it's Charlie's music primarily. And even my own music was an attempt to play more and more unbridled in that idiom. The two songs are my version, my idea of jazz. It may be debatable as to what it is, but certainly the bulk of the album is bebop."

She described tomorrow night's show at Forest Hills as "very eclectic, very textured. We play be-bop, we play rock 'n' roll, there's some choral music, there's a hint of Latin... I don't know what you'd call it. We've hit a kind of plateau now where every show is pretty consistently fun for us."

When the tour ends on the West Coast in mid-September, Mitchell said she may take a brief vacation, then start something new. "I've got dozens of projects, but I don't know which one to jump on first. I just let them unfold."

They will likely include some more of the painting that proved therapeutic during the periods when she was wrestling with the difficult task of setting words to Mingus's intricate melodies. It's her art work that is used on the album they did together.

"I've painted a lot this year. Other years I've drawn a lot or I've taken a lot of photographs. But this year was a brush year."