hiding/masking or renaming domain name that users see

hi,

scenario: company A and company B merge. company A has the better infrastructure and the plan is to merge/consolidate B into A but management would like a new combined domain name 'AB' but it's costly to build new infrastructure.

Is there a way to retain the domain name of company A but all users would only ever see the masked domain name 'AB' at login/authentication etc..

So the oringal domain name will not change but users think it has changed.

Can adding a UPN suffix resolve this?

Thanks