It is sounding more like your faced with a bigger problem. To expand your original example, something like this
if (version == 1)
AObjectModelCollection = InstanceOfSomeApi20.GetSomething(ADouble,AInt,AString);
//Do stuff with AObjectModelCollection in common code from this point forward.
I.E. the web service method call isn't just an interchangable thing. Not sure how to do better than what you already are.