This - sadly.

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.

That's strange, I still have a change primary domain button on my free account, but I've not used it.

Edit: just tried it and was able to change primary domain and then change it back.