[GraemeJ]

Personally, I would be more than happy to pay for the new version and I suspect so would many other users.

Let's face it, this is a whole order improvement over the existing version - it's not just a minor upgrade and I think you should make a charge for it. All other considerations aside (including the fact that you would actually get rewarded for all the work you have put into this new version) the ability to drop back to a known working version would, in itself, be worth the money.

The very fact that this is so different means there is always the possibility of some glitch showing up, no matter how much testing has been done. Many of your users are relying on MS for their work. Not being able to revert to a previous release puts the end-user in a very difficult place, should the worst happen.