What about the rest of the aircraft?
When we took a 747-8F there for cold weather engine start testing about six years ago, they never turned off the APU...
I once had American Airlines maintenance tell me they could remove and replace a 767 engine (CF6) in a shift under ideal circumstances (e.g. at a maintenance base). I'm sure a GE90 takes longer (and doing it at Iqaluit is far from ideal), but I'll be surprised if they aren't done and the 777 back in service by the end of the week.