I think a 767 change is particularly tricky since if it's a Eurofleet route they'd need a CSD rather than a purser (and I don't think all Eurofleet crew are 767 trained) and if it's a mixed fleet crew they can't use a 767 since the crew aren't trained for it.
Having said that BA do seem to have been able to sub in 767s (albeit European configured ones) at the last minute a few times this week to avoid massive delays building up so maybe there is some slack in the system but only when it comes to replacing one 767 with another.