If you don't mind me asking, is it possible you're misremembering? I believe the XT/286 was actually a different computer than the XT, and ran a 286 on the motherboard; it did not upgrade to 286 via expansion board.
The 80286 went into the IBM PC AT. The XT was the one with the 8088 chip. However after the AT 80286 came out, it was possible to get an upgrade card for the XT 8088 that was essentially an 80286 and motherboard on a card. It was like brain transplant for your computer. It wasn't as fast as running a real AT but it was cheaper and a lot faster than the 8088.
This doesn't sound right to me. We had an XT/286 and I don't recall any expansion card involved. Wikipedia seems to agree with me that it was a totally separate model. That being said, it doesn't include a citation for the info, so it's possible it's incorrect.
The '286 expansion cards were not IBM products. they were produced by third parties to give the older XT performance approaching the AT '286 computers.
I have fond memories of my XT/286.