https://support.apple.com/en-us/117267

We have a family Apple ID that exists only because it owns so many purchases. I advise WAITING a few months before trying this. There are likely surprises.

If we do it we’ll be able to enlarge our family sharing; the legacy Apple ID is a family member.

“If there’s anything else associated with your secondary Apple Account that you still need access to, you might need to undo migration. iCloud data from your secondary account can still be easily accessed by signing in to iCloud.com on a computer.”