Set up a separate DOS batch package with this uninstall in it. Then either attach this package to the main package as a dependency package, or when you create your scheduled task, set this uninstall package as a "preliminary", and then your installer as the "Main".. you can chain up to three packages together in a task this way. I would not use the "prerequisite" screen for what you're doing. Its kind of a weird thing. We used to use in the past, and stopped when the behavior significantly changed. It used to be that whatever ran here would run in the context of the logged on user. This may have been a design flaw. Now it runs in the system context. At any rate, the other methods will work better, i believe.
Not sure how you are performing the install of you new package but if you also used a batch file then just combine the uninstall and install - just have a failure to detect the previous version go straight to the installation. That way there is no messing around with dependencies.
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Hear hear! Would be way simpler and more efficient to boot.
I'd wrap this whole thing up in a custom def myself!