Seems odd to me that Christine Lahti, who was doing a fine job with A LOT OF narrative and direct address to the audience, would leave. After such a feat of memorization.
The show was scheduled to end its run in January, but it's extended. Likely Lahti has other commitments. She's been with the show for several months at this point. I've never heard "I memorized so much dialogue" as a justification for staying with a show.