Well...we took down a 721 and older 741, and replaced them with two new 741s on separate controllers...I'm now at around 29TH. When these two older boxes get online in Labrador (we're idling two S7s up there to get the rack space, as they're full up...the S7s will go back online when the new transformer is in) it should still be around 31TH. Then, when their new DC is together, add the 9TH back on...
Life is good.
Since you do have both 721 and 741's you might want to refer to my post in the A7 thread https://bitcointalksearch.org/topic/m.19567389 if you ever think about putting them together on 1 controller. Works well but a couple gotcha's...