The reaction is always the same.
Every time we make a big change to a client's community, people get upset.
Every single time.
You can make the best changes ever and members will get upset. They will get upset because it's different. They were happy and comfortable with the old system. Large scale Digg-like desertions are fortunately rare.
You can't avoid making changes because members will be upset. Members don't have the same data or knowledge as you do. They don't look at the big picture, the future of the community platform, the threads and opportunities in the environment. That's your job, not theirs.
The creator of my favourite video game, Counter-Strike, once told me the following:
"We ignore the immediate backlash. I don't even check my e-mail during this time. We look at our numbers instead. Are members participating more? After a few weeks, I'll begin to check what people are saying again. These are the people who have genuine concerns and aren't just unhappy something has changed"
We embrace this same philosophy. Don't worry about making members happy when you revamp a community platform. Worry about making them more active. If the numbers drop and continue dropping, then you have a problem.