I don't know if UK-licenses are valid for a certain period and need to be renewed (like f.e. in the Netherlands where you need to renew your license every 10-15 years without taking the test mind you).
But if you want to get the motorcycle test (the A-license for 125 cc and up) take it and apply for the French A-license, no need to change your UK license. When eventually your UK-license is up for renewal it's the right time to combine the A and the B(/E) in one French license.
I got my A-license in France and my B/E in the Netherlands, both cards in my wallet and will be exchanged when the Dutch B/E license is up for it's renewal in a few years which means that it will be obligatory changed over to a French B/E license (which is as of then valid for the rest of my earthly days :-))