Changing the case cycle directly is prod is a dangerous task. Remember that all in-flight cases will be impacted. You can move the step 1 from Stage B to Stage A without any impact, provided there is no task occurring in the end shape of Stage A.
You should not do it in prod, its recommended to update in lower environments and perform a full end-to-end testing of both new and in-flight cases.
Newly created cases will work on the new flow but for in-flight cases, those which are in Stage A have to be stage restarted and the data changes already happened in Stage A have to be reset before the restart. Those cases in Stage B have to change staged to Stage A and the data changes happened on Stages A & B have to be reset before the change stage.