This topic was raised in 2020 but it doesn’t look like the suggested enhancement was ever made. I have lots of presentations using old slide masters. I have created a template with a new set of slide masters I want to use instead of the old ones but I cannot see a way to load the new template and make ALL the slide masters available to be selected for more than one slide at a time. It seems I can only use my new template for old presentations by adding a new slide, selecting its Master, importing the new Master Template to that slide and selecting one of the Masters from the set, going back to one of the original slides, changing its Master to the one used by the new slide then deleting the new slide. This is going to take ages. Would it be possible to import the new Master Template without having to create a 1:1 link between a single slide and a single Master on import, so the slide:master links or re-links could be made from the newly-added ‘library’ of Masters after the import?
