I would also love to have this feature for long documents. What I've been doing in the meantime is:
1. Unlock and delete all items on the master page I'm replacing.
2. Copy all items from the new master page.
3. Paste in place on the new page.
Obviously, this creates its own new set of tasks to take care of, but I feel like it's more efficient than manually replacing the applied masters on an 800-page document.
I would also love to have this feature for long documents. What I've been doing in the meantime is:
1. Unlock and delete all items on the master page I'm replacing.
2. Copy all items from the new master page.
3. Paste in place on the new page.
Obviously, this creates its own new set of tasks to take care of, but I feel like it's more efficient than manually replacing the applied masters on an 800-page document.