Old pedals still good after rethreading cranks?
I've been rebuilding my old '74 TdF Gitane. Everything is clean, greased and running fine now. The bike still has its old Ti clip pedals and I want to replace them with Speedplay X3 taken from another bike. The French thread on the cranks is 14 mm x 1.25 while the Sp pedals are 9/16 x 20 tpi. I can get the cranks rethreaded for the Sp, no problem.
My question is : after rethreading, will it still be possible, if need arises, to put back the old pedals without rethreading to french thread ? I fear the new ISO thread will be too big and the old pedals come loose... Or am I just drinking too much Porto ?