Hello! The first SEO expert's rule is not to break something that is well optimised and tuned.

Discover is earned through some kind of classification of the website on Google's side, at the domain level, in a way that you're in, or you're out. Moving the domains in such a way is extremely risky.

It may be that the signals on wich Google is making its classification gets dragged along with the website migration... but you can't be sure.

In the scenario you're suggesting, I wouldn't do the migration and instead just cosmetically rebrand the new domain.

I wouldn't recommend moving Site B to a subdomain... seems like an awful lot of change IMHO ignore the Discover component and continue to improve Site A (it's earned that 'switch' in Google), and see how you can map or integrate Site B content into Site A structure (ie normal migration criteria): Look for performant URLs to map to existing content if very similar or create new content from Site B in relevant areas of Site A if doesn't already exist. As Pedro noted... we don't know more than the Discover criteria of 'create great, relevant, fresh, timely, 'trendy' content, leverage entities, use high-quality images, align with Google News recs, create valuable headlines', etc. etc. Site A's current alignment to get good performance in Discover shouldn't be affected by adding additional great content... but the less you change things the better, and adding a subdomain with lots of new content wouldn't be the way I would go.. but SEO is about opinion as well as experience so YMMV HTH

Changes and migrations are always bound to have some impact on a website. Given everything is migrated seamlessly, no one will be able to predict, let alone guarantee how it will go. Especially with Discover.

I'd suggest only make these changes if you really need to, or if the needs greatly outweight the potential consequences.