Office 365 tenant to tenant migration from a syndicated Office 365 reseller

We are taking over a customer that has an Office 365 subscriptions through their current IT provider.  According to the losing provider, they use a white label syndicated channel partner to resell Office 365 through.  They insist that they cannot provide us with a global admin account or approve our CSP for the customer's tenant.  All indications are that they will have to do a tenant to tenant migration of Office 365.  We normally use Bititian for migrations but according to what I'm reading, they don't support a tenant to tenant O365 migration if the domain on both tenants are going to be the same and they also require a global admin login.  This customer not only has mailboxes but uses Teams and Sharepoint heavily.  Is there any possible way we can still do a simple CSP transfer if we can find out who this company is reselling through or are we going to have to do it the hard way?