It sounds easy enough to shorten the gh. Use the end bows and end walls. Remove as many inner bows as necessary.

Are you talking double wall inflated poly? Poly is the term used by pros for greenhouse, 6 mil, 4-year polyethylene. It has various attachment systems that will remain with the ends and sidewall framing. Attaching poly isn't too hard. Myself and another person have done it on my 32 by 54 by 16ft tall gh.