I've seen and been told of FD coupling leakage after a complete fluid change.
The FD three sealing surfaces....copper bellows seal surface, the FD inner seal surface plus the graphite seal ring can sometimes leak after a fluid change.
These couplings are something you do not want to have fixed especially if they work well and do not leak.
I just had a guy with a C38 woodie that was the type of guy everything should be taken apart and serviced including his FD coupling even though it worked fine.
Now he is in search of a good coupling as a company in texas told him they could fix his leaking FD...they really frucked it up and now it does not leak but has severe vibration after they cut it open to rebuild it.
I do know many people do the fluid change on these couplings (see James Douglas ) spending hours draining and filling them..
I have several cars with FD couplings and have never touched them in over 30 years... they work fine and do not leak.
I have re-sealed quite a few couplings over the years too as the early copper bellows type get pin holed (flexing) and the graphite ring gets severely pitted .
I'ts JMO to leave it alone and be safe if it works good and does not leak..