We had many more switch from surgery to anesthesia but we had one switch anes to surgery during my residency. So it does happen, just very rarely.
Did you match in GS prelim or categorical anesthesia? If the latter, you'll almost certainly have a few GS rotations. Use those opportunities to shine and make friends with upper levels and attendings. Also give Anesthesia a shot - it's a field that's very hard to get a feel for as a student since there isn't much opportunity to contribute. So regardless, work hard during your intern year as you'll need a letter from your PD anyway. See what you think of GS during your intern rotations as well, and talking with upper levels.
To answer you question - it's much easier to switch from categorical anesthesia to GS than from prelim, mostly because the average applicant is much more competitive in the former.