I see that Astronomy beta was merged into Physics, however, many of my questions are now untagged.
Is there some way we can get all the tags on Astronomy moved over to Physics and applied to the questions that were migrated?
Unfortunately, there's no easy way to do this - the migration process intentionally strips tags that don't exist on the destination, to avoid polluting that site with tags that do not belong. In fact, questions currently marked untagged would normally fail migration without moderator intervention due to the lack of any tag overlap on the destination site!
As I've previously noted, all of these questions will need to be reviewed eventually - that's a huge job, but there's no way around it: for as much overlap as they might have had, Astronomy and Theoretical Physics were separate sites with separate cultures and styles; integrating those questions is key to avoiding confusion for future readers.
I've tried to make this a little easier though, by providing lists of all migrated questions along with their original tags. I highly suggest splitting these up into "bite-sized" tasks and tackling them one by one.