You need to read up on major upgrades and perhaps on some of the basics of Windows Installer. Essentially I think you should try to implement a major upgrade, it does involve changing the product code.
Don't be afraid to change the product code. It is the upgrade code that identify related versions of a product. The product code changes between versions. Essentially you author the upgrade table to detect other versions of your product, you update the version number of the MSI(first three digits count), and the package code should always change for every rebuild of the MSI. Finally you must keep the upgrade code the same across releases to make major updates easy to implement.
Installshield shields a lot of the complexity of this if you author the information found in the Upgrades view. Read the information provided here and you should be able to proceed.
Some further information on these important codes in an MSI. You must understand this even if you use Installshield's simplified GUI:
In every single rebuild of the MSI you MUST change the package code. This code should never have been exposed in the whole MSI design - it is used to uniquely identify a file. If you keep this guid the same across multiple files each file will be treated as the same file by definition - even if they are different files. This may cause the most mysterious problems you ever come across with MSI. Using the same package code several times is wrong in every case - unless you want to do hacking :-).
- Package code: identifies unique MSI file
- Product code: identifies product version
- Upgrade code: identifies product family
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