The calendars are in Global.mpt. I'm creating an integrated master schedule, which will contain >100 projects managed by different project managers. I want to accept new versions of their projects, say weekly, and put them into the integrated master schedule.

I would not create the integrated master through copy/paste. Use the SubProject command on the Project ribbon to create the consolidated master. This will preserve all calendars (task and otherwise) and is much less error prone.