The Complications Never end! I'm having great difficulties getting models modified on V5R18 to open with V5R16.... Stupid Dassault and their lack of foresight

I've managed to blunder through some step conversion that have actually worked suprisingly well (Actually functioning in a manner I've never seen step files operate) but I now have one assemblie/part that I can not get R16 to open. I REALLY wish the documentation was better. Am I going to have to get 3rd party converter? I can understand R16 not opening R18 files but I Can't understand R18 not being able to save in an Rx compatable format....

Sorry for all the questions but I really do appreciate the help.