Originally Posted by
DL-Don
I've had many MX subs over the years. The vast majority were same equipment subs. In cases where the reason for the sub was not explicitly stated as MX, they have always been a downgrade--- CRJ 200 for 700/900, ERJ 145 for a 170/175, nonlay-flat for a lay-flat, etc. Last minute MX situations I understand but when it is a sub several days in advance it is much more likely part of their bait-and-switch scheme.
It could come off nefarious but I bet a more likely reason is maintenance that pops up. Or any number of other reasons. Could be a mechanical issue that they hoped to fix at the hub but turned out a part needed to be ordered and come from Seattle. 1-2 days for it to arrive and your flight was the 2nd or 3rd day.
As most have said subbing a downgrade for an entire cabin isn't something most airlines would like to do. Someone sitting up front knows what they booked and likely expects a certain product. So intentionally doing it over and over again wouldn't really work out for them very well.