I knew an Italian AF FW (AMX) pilot who had been given a RW course, then sent over to the UK to do the ETPS-RW-TP course. After graduation, he went home to work as a TP on both.
UK-CAA's new FW-light aircraft TP is an ex RW-TP with a FW-ATPL. So far, he's doing an excellent job of it despite nor having been formally trained in FW test flying.
One of the UK's main sport flying associations (BMAA) has a CTP who used to be CTP of BA(Helicopters).
So, crossover doesn't seem all that unusual. Mind you, I can't offhand think of anybody who has gone from FW-TP to RW-TP? Is that simply that by proportion, there are already more than enough RW TP's in the world for the finite amount of helicopter test flying that needs doing perhaps?
G