I’m sitting in J on AA39 LHR-MIA, which today is scheduled to land at 3:22 pm.

I am flying MIA-SJU on a separate PNR on AA316 at 9PM. Now, that is a long layover with an even later arrival in PR.

I can get request a same day stand-by for the flight leaving at 5:44 pm, but I’m unsure whether it’s wise.

At which point would they usually confirm the change? And if they do, would I immediately lose my seat on my current flight?

What happens if I’m delayed at immigration and customs and I miss the new flight?