I would recommend keeping it simple and not updating the code for “Open issues without Story Points” Consider building a new calculated measure and referring to “Open issues without Story Points” using a tuple construction:
--all open issues without SP
([Measures].[Open issues without Story Points],[Issue Type].DefaultMember) -
--except for Open epics without Story point
([Measures].[Open issues without Story Points],[Issue Type].[Epic])
To learn more on tuple constructions, you might check out the documentation and training video: