"If it's planned maintenance you'd think they'd have replaced the original plane with one that could provide customers with what they booked wouldn't you? If there was an unexpected hitch, and that's the only plane available it might be more understandable but to use something like that when it's been planned seems ridiculous to me, but then what do I know?!!"
But who is to say that a like for like aircraft is available whilst the original aircraft is on maintenance?
Or maybe if one is available, it is costing so much more to lease in that the airline would risk pissing off a few passengers with a less welcoming machine that costs less.