I too got a lot of flack from my membership when I switched. Mostly from a vocal minority who quickly adapted when they realized we weren't going back.
As has been mentioned, take some steps to integrate the new software into your community. Get the style as close as possible to the old style. If your old community didn't have side bars, make sure the users have the ability to turn them off in the new software. I also set up a test board before hand and invited my mods and some distinquished members (read: Post whores) to log in and provide comment on what features to use and which to turn off once we converted.
I still get the complaints about the few technical issues (search, PMs etc). The complaints about the "cramped feeling" have all gone away. Those who don't like the side bars have them turned off.
My community is a mix of young and old (teenagers all the way up to 70+). The older folks (I'm right there in the middle at 43) were probably harder to convince that the change was a good thing... but they did come around. This after having our community stagnant in UBB.classic Version 6.x for many years as we awaited the "next generation". So the membership was extremely comfortable with the old software.
I guess my only remaining advice Basil, is to say that it is your community. The stewardship of a community isn't an easy duty. You won't please 100% of your membership, even on a good day. I've been called every name in the book over the last 7 years. It's all part of the job. If the vast majority of the community sees that you're making changes for the good of the community, they'll come around. Unfortunatly, reverting back to the old software has taken the control of the community out of your hands and placed it into the membership's collective hands. Take back the reins. Take the steps to reduce the shock of the transition... then buck up and press forward. They will come around in the end and see just how much of an improvement the new software is over the old.
If you need any advice going forward.. we're all here to help you. After all.. we're all in the same boat so to speak.
Good luck!
