I think I understand the question. You're asking how to deactivate a key and then use another key for the activation, correct?
If that's what you're asking then the answer is to use the Deactivate() function in TurboActivate. Specifically pass "true" to the Deactivate function to let it know you want to Deactivate and remove the product key (instead of just deactivating without removing the product key).
Or the alternative is to deactivate like you're doing right now, and then instead of calling Activate() again first call CheckAndSavePKey() to save the new pkey you want to use.
Either method will work.