How late can I do a same day flight change?

Sometimes I'll get to the airport a little early and there's a flight leaving earlier than my scheduled flight that has open seats and hasn't closed boarding yet so I'll just change my flight and get on the earlier flight. I have premier gold so this doesn't cost me anything. Is there a cutoff time (for the earlier flight) after which I can't do this? I have to re-check in after changing the flight, and does that mean I have to complete the flight change at least 45 minutes before the earlier flight leaves since the check in cutoff is now 45 minutes?

The last time I did this it was something like 47 minutes before the earlier flight's scheduled departure time, and in hindsight I'm not sure if I almost got myself in trouble by cutting it so close if I switched flights and failed to check in by the 45min mark.