I can't say whether installing over an existing version will "wipe out" your existing data, but even though I'm inclined to think it won't, it's safer to assume it will in the absence of any firm evidence to the contrary. It also doesn't help that the Incredimail website customer support is non-existent. That alone would deter me from using it. However, it's your decision.
You may find that an older version won't install over a newer version anyway (only guessing here, based on the behaviour of many other applications.
If that's the case, you'll have to uninstall the current version first.
With that in mind, I found the following information on how to back-up your existing data and restore it to the older version when it's been installed:
http://email.about.com/cs/incredimailtips/qt/et111002.htm
Hope it helps.
Pip
This post has been edited by pip22: 28 February 2007 - 09:47 AM