Yes, you can issue a short year statement & start a new computation year. That will be confusing to many customers because they may have just received their annual statement and now get a short year statement with only a month or 2 on it. If you choose this method, you're not starting a new account. You are simply cutting off the historical info and starting a new year of historical info. Thus, the previous year projection would not be included at the end of the first (new) computation year with the new provider. Only info since the conversion would be shown. You still have to give the prior projection, so how is that going to transfer to the new system? Maybe you can print and scan those. Then when you get a year out, you simply give them a copy of the projection - much like a 1 year old escrow and you give them a copy of the initial disclosure (some banks do it that way).
I hope this makes sense. If not, post again and add dates as an example. I'll try to follow up if it's not clear.
It's too bad the conversion can't grab the historical info (but you know that).