as long as you put the filling hose below the surface of the older beer (like sitting on the bottom) and keep the pressure low, I wouldn't think it'd foam up as long as it's the same temp, if you're mixing temps, that will probably cause a lot of foam...
That government is best which governs the least, because its people discipline themselves. Thomas Jefferson
On Deck: Bavarian Hefeweizen
Primary #1: Edwort's Apfelwein
Primary #2: Green Flash West Coast IPA
on tap #1: Addicted Coffee Stout
on tap #2: none :(