I had an old installer which was a basic visual studio setup project however after some time we realized that it was just no where near as complex as we need. So I've spent a few days generating a replacing with WiX.
Now my current problem is upgrading from the previous installs. If someone has installed one of the versions that was installed with the VS Project, is it possible for them to use the new WiX project? I have tried with setting the upgrade codes the same but it's not making any different, it seems to think its a completely new product and tries to install again.
Is it possible?
See the help topic: How To: Implement a Major Upgrade In Your Installer
Major Upgrades can only perform upgrades in the same installation context. (Per User -> Per User or Per Machine -> Per Machine ). If your VDPROJ was installed Per-User by accident then the user will have to uninstall / reinstall to become Per-Machine.
Your new installer doesn't have to have the same UpgradeCode as the old installer but it does have to author the MajorUpgrade using the UpgradeCode from the old installer.