DVD Universal Music put out a few years ago...

What they mean by "what we thought was 'Diana Ross & the Supremes' was really 'Diana Ross & the Andantes'" is that, as opposed to actually using Mary and Cindy to sing the backups on the singled recorded after Ross was given top billing in June 1967, the backups are by the Motown in-house session singers The Andantes.

The Andantes (Google them) sung on thousands and thousands of Motown records, singing backup for Marvin Gaye ("I Heard It Through the Grapevine"), The Four Tops (all of their major hits), and so on. Motown had already been using them to mask the fact that the Marvelettes couldn't harmonize, and now that Gordy wanted to take Ross solo, he was desperate to (a) come up with a hit single for Ross and (b) make sure those pesky Supremes weren't on it.

Mary and Cindy still appear on some of the album tracks, and on everything the group did on their Temptations duet recordings (such as "I'm Gonna Make You Love Me"). But the Supremes' own singles:

Forever Came Today
Some Things You'll Never Get Used To
Love Child
The Composer
I'm Livin' in Shame
No Matter What Sign You Are
Someday We'll Be Together (with backups actually by the Waters sisters, not the Andantes)

are essentially Diana Ross studio recordings. And that list actually comprises all of their singles released in 1968 and 1969.

When she finally went solo in 1970 ("Someday We'll Be Together" was supposed to be her solo debut, but given that title they couldn't help but use it as a Diana Ross & the Supremes swan song), there was no need to change personnel. Business as usual.