Ah, now you've caught me out there.
It's been a while since I used Activesync but, IIRC, you need to follow these steps (it's actually called a partnership not association, you can see how long it is since I've used it):
Before connecting your device, delete the old partnership on the PC: Open ActiveSync on the PC and go to File/Mobile Device and select the partnership you wish to delete (it's the one where there is a "bullet" in front of its name in the menu).
Then go to File/Delete Partnership. (The prompt window will confirm that the backup will not be deleted just the partnership). Then, re-establish the partnership by connecting your device. ActiveSync will automatically suggest that you establish a new partnership and will prompt you through the process.
You should, if possible, backup the device before going through this routine. I don't know which version of ActiveSync you've got but that was the menu structure on the old 3.7 activesync that I used.