Neglected to report this one when it happened, but 2018-10-14 ORD-COU (connecting from LGA) and unknown (to me) IRROPS caused them to switch from a CRJ-700 (9J+56Y) to an ERJ-145 (48Y). I had previously been confirmed in a J seat around T-48 with a complimentary EXP upgrade. When I went up to the desk for a seat assignment, they immediately gave me a $300 voucher (along with the seat assignment!) because the new plane didn't have J. I don't know whether that was proper or they made a mistake and didn't notice I got the J seat for free in the first place.
Either way, then they asked for volunteers to take a later flight for $530. Their intention was the next COU flight, which was the next morning; instead I asked whether they had J seats on the STL flight 40 minutes later. They said they did, and I volunteered to take that flight in J and the voucher. Sadly I had to give up the previous $300 voucher, but as a bonus the flight earned as true full-fare J, and the $530 paid for nearly all of a flight to Peru next year. Since I was going right back to NYC in a few days (one of the few places there's an AA direct from STL) I just hung out with some friends in Soulard and flew back out of there STL-LGA-ORD-COU.
Next time I'll see if they'll just drop me in SAN or something...