User communities don't want to see things change. As long as the community is up and running all the time and is very fast and responsive, they are happy. Regular users invest a lot of time in your community, and they become adept at using it. Any time you make a major change to the user interface, they will scream, loud and long.

Sometimes you have no choice but to make radical changes. This usually happens when a company is acquired, and its community is merged with the new parent, forcing an interface change. The result can be more than unhappiness; you can lose key members of your community and affect membership and growth for an extended period.

So, short of an absolute requirement for radical change, the best option is to make incremental changes. This lets you improve the user experience and add functionality, while you monitor the user feedback to make sure it is working. You can work towards a significantly enhanced experience this way, while keeping your community behind you.

If you have to change vendors or technology, the best option, if you have a vibrant and happy community, is to copy the existing interface, and then continue making incremental changes after the conversion.

If you have to change your place radically, though, do make sure that you let your users know well in advance. Make sure they understand why the changes are being made, and try to get your most prolific and important users to support the change.

No one likes to be forced to change, even when the result is much superior. So as you work towards providing a better place, remember to keep your users happy with incremental changes.