Zone delegation to GTM/DNS Question

We have a customer whose DNS zone is hosted at an ISP. Say the zone is Customer.com

 

The customer has many hosted websites at his own premises and he want to migrate to GTM, but not all websites at once, just one after the other.

 

I was wondering how I would be able to do that without changing at all once, I was hoping someone might have doen that before but I have a suggestion, it would be great if I get feedback for that.

 

I was thinking that maybe we ask the ISP to create a zone called, internal.customer.com and for each website the customer has, say for example eservices.customer.com, we would ask the ISP to create a CNAME record that translates eservices.customer.com to eservices.internal.customer.com And then we would delegate the zone (internal.customer.com) to the GTM, and for each new A record we want to move we would do the same. Then on GTM we would create a zone called internal.customer.com and a wide ip A type called "eservices.internal.customer.com" that has an Alias of "eservices.customer.com" I am not very familiar with DNS/GTM setups but this is my suggested approach, is there an easier way for a smoother migration and easier rollback?

 

Thanks in advance.