Do we know if this cast is doing the entirety of the tour, seeing as it is a very long tour. The old tour of the play used to change the cast up quite regularly. For some reason I can't imagine Fern Britton to have committed to the full 60 weeks.
I'd be surprised if the entire cast was contracted for all 60 weeks, a lot would depend if there were many breaks in between as I wouldn't think Ruth would want to commit long term to something like that and others may want option to take on other work in due course.
A lot will become clear when Theatres on the tour up date their website with casting details.