The online contact database service from www.plaxo.com is great to help me keep sync with my contact in Exchange/Outlook and Thundbird. Once you setup an account, you can install a plugin into outlook and thundbird so any change you have will be synced through the web based service.
But you need to take extra precaution when you re-install your outlook. I once have plaxo just created all the entries in the online database into the outlook/exchange database when I re-installed outlook client. All the contact items just doubled to two exactly same items. As I don't want to pay the premium service just to fix this kind of duplicates so I had to go through around two hundred items to remove the redudant one. I like redudant often as I design my network but not this kind. :-) After that, I deleted the contacts folder in plaxo web site and re-sync again.
So when you re-install outlook which will retrieve contact from exchange server, just delete the contact/calendar etc in the web site first, then you can install the plaxo plugin, sync up to the web server.
Or you can delete all contact/calendar etc in your exchange server first, then install plugin to pull data from the web server.
As a good practice, it's always good to backup data using export from plaxo web site.