Software Deployment

Currently, my package writes a value into the registry using the [ISX_SERIALNUM] value collected at install time. I'm keeping this in my program's section of HKLM\SOFTWARE. When a user who already has the app installed runs a newer version of the install, it starts this upgrade process (which seems to be default behavior, as I don't believe I specifically asked for it). The [ISX_SERIALNUM] value is not collected during this upgrade, thus the value in the registry gets waxxed.

I have 2 options.

1.) Ask for serial on upgrade (how?)
2.) Keep the package from writing the value unless it collects it (how?)

Thanks for any help