No need to "side-line" your amp... just throw that spare 8877 in and run it!
Also, being that the 8877 (3CX1500A7) is a ceramic tube, there's no need to "cook" it in or do any sort of "degassing", or anything else.? They are not prone to leakage, as a lot of old glass tubes are (like the 3-500Z's).
Tube swap is very easy, only takes about 30 minutes...? Removing (and reinstalling) all of the cover screws takes longer than the actual tube replacement. LOL!? Pull that old one out, plug the new one in, hook up the anode lead, put on the cover... and you're set.
I swap the tube in my big Henry 3K-Classic fairly often (I go between an 8877 and a 3CPX1500A7, pulse rated tube).? In addition, I just recently swapped-in a spare Eimac YU-158 (a newer version of the 3CPX tube) into my Alpha 9500 (pulled the OEM 8877 out) just to exercise it.? I'll run it for a while, then pull it out and go back to the 8877.? No problems whatsoever.? The Eimac YU-158 tubes, as well as the Eimac 3CPX tubes, come up for sale on eBay fairly often...
As long as your no-drive, idling plate current is good (which it should be, if your spare tube is good), you shouldn't need to do anything else.
73,
~Alan
W5ARM
Alan - W5ARM