To sign in, use your existing MySonicWall account. To create a free MySonicWall account click "Register".
Best I can think of - without any research - is to go into MySonicWall, click on Product Management and select My Products.
Select a device from Tenant B and click Transfer Products (right side of lower menu) to send it to Tenant A.
When done moving all, delete Tenant B.
Hope that helps.