Oncemorewithfeeling summed it up!
Kelli always is trying to do something new and switch things up. Which is why she left Piazza to do Pajama Game to showcase herself in a completely different role. She went from Nice Work to Far From Heaven to Bridges of Madison County to King and I all within 2 years. These are all completely different roles. The only Broadway revivals she's done are King and I, South Pacific and Pajama Game (unless you want to count Follies in which she was only ensemble and an understudy).
No one has confirmed she's even doing My Fair Lady and if she does, I would bet good money she'd do something different in her time between King and I and MFL, like a play or something off Broadway. She likes to switch it up after each production, though if and when Julie Andrews calls its probably best to answer.