Ok firstly, any tube change should be accompanied by a bias tweak. Even if the tube is in the same family or indeed the same type of tube, there will still be differences. Most companies who sell tubes claim to balance all their matched sets evenly, but it still helps to tweak the bias. Its not gonna hurt it if you do a straight swap with EL34/KT77/6AC7 but its like air pressure in a tire. Needs to be just right for the amp to run as well as it can.
Phano36 has pretty much said all the things I would say. I will stress again that EL34 to 6L6GC is a far more complicated switch and could require a new output transformer amongst other stuff! If the tubes are in the same family then it should be safe to swap between them.
As for having the amp cranked, I play alot of hi gain stuff (I use a tube screamer in front of the amp most of the time, so lots of pre amp distortion), and it always sounds better cranked (therefore introducing power amp distortion). You don't have to crank it to get it sounding good....but in my experience it always sounds better if you do, even if power amp distortion isnt your main goal....seems to just be the way tube amps work :).....and rightly so!