Not sure about the other models, but for the -7(a) You have to commit when you order the fuselage kit. Of course, it does seem feasable that one could change his mind after receiving the fuse kit and just send the parts back for the parts specific to the other model, but that's a waste of time and money...so the fuselage is the only real viable decision point.
Of course, the -7 demonstrator was converted to a -7A, so I suppose you're not really commited to one or the other...but that's major surgery I would not want to perform.