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Yes, they can force you back onto active duty and court martial you. That's the worst they can do.
Now, will they do that? Almost certainly not. It takes too long, requires much to much effort and probably won't bother. Most likely though they'll work to have your characterization of service changed, which they can attempt even without your presence.
They can change is based on the failed UA. They don't really need much, in terms of misconduct, to change your service characterization.
Even one issue of misconduct in a service period of 8 years is legally sufficient to change your characterization of service. All they have to do is submit an amended DD Form 214, with a general discharge characterization rather than an honorable, if that is what you were getting.
As I said though, it is unlikely that they bother to do anything if your paperwork is already completely done and you are gone before the result return.