I did this twice last year on RDU-NYC routes where I was ticketed to JFK and when I got to the airport I asked at the AC if I could standby to LGA. Both times I was put on an LGA flight at no charge, once I scored an immediate upgrade to first. In both cases I was on American Eagle flights but I don't think it would've made a difference (a handful of the RDU-NYC flights are mainline but on A319 so precious few FC seats on those). I'm EXP, and this will be a YMMV situation particularly if you have low or no status.