After renumbering the switch stack, I have the answer to my question.
When renumbering stack members, the interfaces applied to the interface groups do not automatically change/move to follow the new number of the stack member. The ports stay applied to the interface groups.
If renumbering a stack member to a number that didn't previously exist, the ports on that switch will change to a default config. You'll have to add the ports back to the interface groups they previously belonged to.
Moral of the story is, plan ahead. If renumbering multiple switches as I did, be prepared to clean up (remove) ports from interface groups and reapply them using the updated stack member #s.