Changing your partner domain or "tenant"
We've been a partner for > 15 years and during this time the company domain has changed a few times, this didn't really cause many issues but with the move partner center it now seems that everything is locked to your default partner domain, office 365, visual studio etc. A real pain if your user accounts dont match your partner domain.
Is there any way to re-set you partner domain (or tenancy ?) otherwise partner site assumes that all users will have an email or microsoft account using the default domain.
Re: Changing your partner domain or "tenant"
Partner center only uses organizational accounts (managed in AzureAD), it will no longer accept the personal Microsoft accounts (Live IDs). Your MPN status is mapped to the tenant you choose during migration.
E.g. when migrating from old MPN site to Partner Center you used "email@example.com" as account where the MPN status should be mapped to. So the tenant this is user is from with your custom domain mydomain.com is the tenant where MPN status is registered, and you can only use accounts from this tenant to log in to Partner center.
However, you can still register additional domains in your tenant if you want to - e.g. if you add another custom domain like myotherdomain.com to your tenant, you can set up user accounts using this domain. You can also choose that user need to use the default tenant name - e.g. firstname.lastname@example.org if your prefer.
What can not do anymore is that users login using the personal account - e.g. email@example.com, firstname.lastname@example.org etc... And it might be more confusing when users have registered their work email before as Live ID - so email@example.com is also a valid LiveID/MSA - users may get prompted to choose "work" or "personal" account in some locations, but not all.
To your questions - by opening a support request you can ask to reset the migration state, this will take a few weeks and then you can choose again which organization account & AzureAD tenant to use when repeating the migration to Partner Center, but again only users from the chosen tenant can be allowed to log in.
So, if I understand you correctly, the right choice is rather to register a new custom domain in your tenant whenever your company name changes and set the user accounts to use this other domain upn.