Keynote master slide problem

I have a Mac Studio, using Sonoma, and Keynote 13.2.

I have a Keynote presentation, with both old and new slides including master slides.

I would like to click on an old slide, and apply the master slide background.

I cannot figure out how.

My only solution is to create a new master slide, and manually copy and paste dozens of slides. Tedious.

I saw a discussion in 2013, and hope this problem has been solved. Is there a way?

Thanks.