If the airports in question are on the official co-terminal list then ...
1) how far in advance? I assume you're referring to same day confirmed. Anything further out than this would be a ticketing change and would depend on the rules of your ticket (typically $200 change fee + fare difference). Same day confirmed can be done 24 hours in advance of your original flight.
2) Same day confirmed is confirmed. Yes. And you are back on the upgrade list. The only thing that I can think of is if you cut something crazy close and did a change < 1 hour to departure (not sure if they'd even allow it, but maybe you showed up at LGA by mistake and convinced them to switch you). In which case maybe the upgrade list would have already cleared...
3) I'm not 100% sure, but yes, I think it works.