M2Dude and I seem to be forever crossing posts!
The only time I recall that we consistently had a stby aircraft nearby was Saturday morning, when the BA001 machine would be parked next to the BA273 (BGI) machine. The BA001 could theoretically stand in but I doubt we'd have done it.
I have done a ship change when we found a problem with the front-runner (can't remember what it was but it was one of those 'hard to determine an exact cause' issues found in flight-crew pre-flight checks) and even though the spare was on an adjacent stand, as M2D says, it still took an hour. (The delay, of course, was less than this as we found it early enough).