I don't know, but I feel like "Lucy" was a better choice than "Ah, But Underneath" for Jan (and I'm not just saying this as a fan who likes "Lucy" more).
I feel like "Lucy" gets more at the duality of the character more, how she struggled with herself to go from "plain Jane" Phyllis to sophisticated and smart Phyllis better.
I always felt that, although "Ah, But Underneath" is a fun song, the final message of the song is that Phyllis spent all these years building up walls and facades to her life, but when you strip everything away, there is nothing left. And to anyone who saw the Kennedy Center production, there is a lot left to Jan's protrayal of Phyllis, and she is actually the strongest and most self assured of the four leads in the end (her "Lucy and Jessie" combined).
I never got around to writing my review of the show on the main "Follies in DC" thread, but I thought that Jan actually didn't do to bad in the dancing for "Lucy and Jessie". I thought that Ron's choreography was actually worse and more distracting during "Live, Laugh, Love".
Anyway, congrats to Jayne and Mary Beth for joining the cast. I'm also hoping for a return to the montage ending (and the ghosts) for their number! And a recording of any sort (cast album or a PBS/DVD release).
Xanadu! Can't cry on cue!