Master Page assigments don't stick with pages (FM 8)

I inherited a book done in FM 8 that looks correctly structured and formatted. This book has about 25 chapters, and I need to add new chapters wtihin the existing sequence.

On the Reference Page, the ChapterTitle tag is associated with the "First" master page. I can't find a way to create a new section starting with the ChapterTitle tag, which doesn't throw off all the other master-page assignments.

It's tedious going through the whole manual to reassign all the chapter opening pages, and then finding and reassigning all the pages that are not chapter opening pages, which have been bumped to the "First" master page. I think the "First" assignment should move with the ChapterTitle tag, and it should also move with a page that it has already been assigned to. Is there a way to make the assignments stick to the pages that already have them?