Is a re-implementation necessary to change their calendar in their app? or can they just change it? They're on monthly, would like to stay on monthly, but change from calendar month to 4-4-5 month

Once you add the custom month start dates if applicable you will get the right number of days in a month when using the api.days functionality. if that api is the only thing that the Fiscal Year Month Type affects, and they're currently not using that in any rules, I would suspect that re-implementation is not necessary.