FCC Certification of same electronics in a different size enclosure
Short answer: you probably don't need to have it re-tested.

Long answer: Testing requirements for changes to already certified devices are covered by the FCC's permissive change policy. 

The re-test requirements for enclosure changes depend on the size of the changes. I would ask a TCB or the FCC directly to get confirmation for your specific case. It's a little subjective so you may get different answers from different sources. 

I wrote an article called "Is a design change without a re-test too risky" which covers the case in much more detail.