My understanding of this is similar to Hilltop's: Assuming that the bias circuit has been modded, you will generally be ok with any of EL34's,6CA7's, KT88, 6550, KT66, KT77 & 6L6GC in any amp designed for any of the above, with the exception of an amp designed specificly for 6L6's or KT66's.
In a 6L6 amp, as Hilltop said, pins 1 & 8 are tied together and would need to be separated, but the bigger issue for 6L6 amps running the other tubes is the heater current draw. 6L6's draw considerably less heater current than all the other tube types, and if the power transformer is designed ONLY for 6L6's and can't handle the increased heater current draw, your gonna have problems.
The issue with KT66's is the plate to plate impedance, where an amp designed around KT66's would have an output transformer with a primary impedance of around 8k/pair of tubes, where all the other tube types would be looking for approx 3.4 - 4k primary impedance / pair. You can usually get around this by mis-matching your cab to the speaker output on the amp, so running KT66's in an EL34 designed amp, or vice versa, is not usually a big deal (other than biasing)
One other thing to keep in mind when switching to the big bottle 6550's or KT88's is the plate dissipation wattage you use in your bias calculation. For example, 6550's are rated @ 35watts, and most KT88's are 42 watts. Biasing these up to 60-70 of their rated power is going to put more of a load on the power transformer than if you use the 25 watt figure for EL34's.
Bogner says it's no problem with their Rev Blue to run KT88's properly biased, and their power transformers are likely very high quality,
But... be aware that with some amps, this MAY be an issue. For example, there are stories out there where the Marshall 2203KK is designed to be biased as a 25watt per tube amp, and if you crank up the bias on the the 4 KT88's to dissipate 42 watts per tube, the power transformer Marshall uses in these amps can't take the load at high volumes and can overheat and fail.
Stay away from doing 2 hour sets in stadiums and you'll probably be OK, but YMMV.