Office 365 Tenant to Tenant migration but only mails

Hello,

I am not sure if i am in the right section.

We need to migrate our Office 365 to a new tenant, but we only need to change our mail domain name and we need to keep ALL the rest as it is. ( we absolutely need to keep our Sharepoint for example ).

I am not sure if i  explain it right.

Tenant A acquired Tenant B.

We are both in Office 365.

We need only to change domain name mail ( for external client ) from Tenant B with the Tenant A domain name.

no change for the data, Azure Servers, Sharepoint, OneDrive, etc... ( and off course, we need to keep all our rights ).

Is it possible ? easy ?

if yes, any suggestion how to proceed ( or a website link ? )

Thanks in advance for your help.

Nicolas.