Migrating to Google workspace

I just purchased the add on, workspace.  However, I used the same email address I had for Roundcube (Rm).  Now I am having trouble accessing the old Roundcube email and I can't migrate the old RC emails into my Google Workspace email.  

Was I supposed to use the temporary admin workspace email, migrate emails to that one, then create the workspace email that's the same as the old roundcube?

How do I migrate the old emails?  The posts on here are not useful.