You may want to put your AC status in your profile (AC*P etc) so that advice can be given accordingly. With Prestige, you don't have automatic access to the lounge. Technically all rules are subject to your original fare.

However, anyone (even no status) traveling on an upgraded ticket is currently given priority check-in, executive bag limits, and priority boarding. The lounge is the one exception, and given that your boarding pass will say AC*P, chances are the dragon will pull up your original fare class and you won't be allowed in. This restriction only applies to MLL, as other *Gold lounges will let you in with a J boarding pass without hassle.