they must be interchangable, because they are both applied to estp, and estp test as estp in both systems.
if they are not interchangeble, one of them must be untrue.
i dont know you, but for the isFp i know, i can tell, they dont have the same S that estp have, but they have Si
I see what you mean, but as to which is untrue I do not know.
See here: Sensing logical introvert - Wikisocion
Socionics states my preferences are Si/Te/Ne/Fi, while MBTI states that they are Ti/Se/Ni/Fe.
So one of them has to be incorrect, but in order to determine which one I need concrete definitions of Si/Se/Ti/Te/Ni/Ne/Fi/Fe, and I have not been able to find anything consistent.