Your UK CPL is a different number to the PPL. The whole reference makes up you number not just the numbers. So a change from a PPL to a CPL invalidates the 61.75. When your CPL is changed to a Part FCL your number will change again.
You can't be issued an FAA CPL on the back of a foreign licence. You just get private privilege regardless of the underlying licence.
So if your 61.75 is invalidated through changes to the underlying licence you must go through the issue process again. Get the CAA to issue the validation letter and arrange a FSDO appointment. Once done, get a BFR and away you go.