I suppose one thing we are missing here is that Lis probably knew she was ill and probably discussed what would happen if she became too ill to work. If they did indeed film the first and last espisodes of the next series first in the run, perhaps she was concerned that she might become too ill to work and wanted to be sure that the show reflected her departure. She was a true professional and I would anticipate that she had a hand in whatever decision they make for the sake of the cast, crew, children, and adult fans.