I only know about POCO substations, so my opinion may be extreme, but....why risk an on-line changeout? Too much risk of losing protection or false tripping. Maybe the private sector is different. The best thing to do is take the station off line by switching loads to alternate circuits, open all the DC loads and relax. No stress in an off-line sub. They say they can't take it off line, but it's usually just because it takes time. It can be done....maybe it's just me though. I've tripped more than one sub messing with the DC. Even using a backup bank (if you can create or find one), there's always a time when you are unprotected. Substation faults can be pretty impressive.