I just want to give me personal experience just couple days ago. I have a confirmed 6:45pm ORD-LGA flight with one checked bag from international itinery (TPE-NRT-ORD-LGA).
I got my bag and recheck it right after the security. And I told the agent that I want to standby for eariler flight (4:30pm ORD-LGA). He warned me that I might not be able to catch that flight since I have to get back to security line (3:45pm at that point). However, without a doubt, he changed my laggage tag, and check for 4:30pm flight. And he told me worst case senario, the bag will arrive 2 hours before me landing at LGA.

Eventually I made the standby (arrived at 4:00pm, and they just start boarding) and got my bag on time with eariler flight.