What do you do with the old license when it is upgraded to a new license?

In a previous post, we discussed how to allow users to freely access minor upgrades while ensuring you get paid for major upgrades.  But had you thought about the likelihood of a customer purchasing a discounted upgrade and continuing to run BOTH versions on two different computers?

Is this a violation of your license agreement when you sold them the discounted "upgrade" license? Should customers be allowed to do this? What might be bad about you revoking a license?

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