Nope, it won't reduce your lifetime miles. The counter only goes up.

Similarly, if for some reason you return something that you earned AAdvantage E-Shopping miles on, you'll lose the miles, but they won't be deducted from your lifetime counter.

This presents the interesting situation where, if your only goal is gaining lifetime status, rather than using miles, that you could swap miles back and forth a few times with Hilton, Diners Club, etc... you'll burn through them very quickly, they'll lose value very quickly, and you'll get nothing tangible out of them, but it might be enough to get you over that 1,000,000 mile threshold.