Perhaps they want to do something procedural like IR or even anesthesia and feel they'd benefit from a GS prelim moreso than a medical one. Another thought is maybe they're reconsidering surgery as a career path altogether and want to get the foot in the door with the prelim. If it's the latter, just know OP that you won't seamlessly flow from prelim surgery to PGY-2 GS and you're going to have to show up to your med prelim July 1st so there's no urgency or something to really plan right now. If you have a good application (Step scores, clinical grades, etc.), you may just find it easier to NETWORK now, do the medicine prelim, and apply to categorical surgery positions this fall. You also have two months right now to do surgery rotations so I would make sure you find a way to get two months of surgery electives in for letters. Anyways, OP intentionally chose not to specify ("for my own reasons") so seems like they just want a feasibility assessment.
In that regard, I'm not sure. Obviously whatever you want to do will have to go through the GME offices at your incoming program and outgoing program. If it's feasible (via a switch), I'm guessing your best prelim surgery shot would be at the place you're already doing prelim medicine given the program directors may have a relationship moreso than a medicine PD and surgery PD do at separate institutions. I think (but don't know) that this would likely have to be after you show up to work on day 1 in order to fulfill your match obligation so definitely start making living arrangements for the medicine prelim. If you're looking on changing fields entirely and moving to surgery, it's even more complicated with your advanced position having to get involved too.
