Right now there's a section in prime video that's titled free with ads(only if you have prime). After the change, all prime videos will be in that section.
'free with ads' is literally free to watch, with ads, no prime membership requirement. a prime membership won't shed the ads there now.. or next year. it's a different selection of programs or a separate service (like freevee, aka imdb tv).
the 'channels' you pay extra for now? that'll probably be up to the individual provider of that programming and their future deals with amazon to be on the platform.... but the infrastructure for them to add ads is there.