I also own a legacy account but google really wants you to stop using it.
- At first you were able to just change the primary domains / add new domains at will - Then it was only possible for full gapps accounts. There was the trick to upgrade to the trial, do all the domain changes, revert back to the free tier - Then google changed that as well. Adding a domain in the trial will block you from downgrading until you remove the domain again
I think you can still change the primary domain by upgrading through a gapps partner, but then it's not a legacy account anymore.
Edit: just tried it and was able to change primary domain and then change it back.
Edit: I just remembered reading an article [1] about hooking up a regular gmail account to the free tier of mailgun, so you send and receive through your custom domain but otherwise have a plain old gmail account.
[1] https://simplyian.com/2015/01/07/Hacking-GMail-to-use-custom...
wow, this is really a thing.
It'll never be used but I'm glad I can say I have it.
(edit: what I mean is that he can recall the account and get full access to it while DNS still points to Google)