They should indeed allow the switch of <ourproduct.com> to our .onmicrosoft.com tenant (instead of the previous domain name holder) but unfortunately this is a manual process as demonstrated by my experience.

We were indeed asked to add a TXT record (ms=....) to our domain which we did but this was not enough to be able to link the domain name through the admin panel ourselves because it was still linked to another tenant which held the domain name in the past.