It means that videos by "partnered" creators who had their videos on the old ad-free site will no longer be shown, until they are migrated over to the new ad-fr

It means that videos by "partnered" creators who had their videos on the old ad-free site will no longer be shown, until they are migrated over to the new ad-free site by agreeing to the deal. The sentence in the article is just ambiguously worded.