Standby questions for LHR-MIA and MIA-SJU today (24 Jan 2019)
 

Go to CS desk AFTER you clear immigration/security and see if there is still time to
make it to new flight....If there is any availability you should be able to get on. This
is the safest way....

Thank you! I was talking to a fellow passenger who claimed that a non-confirmed stand-by (as in when the app says explicitly that it's stand-by only) won't "clear" if you're not at the gate by the time they call you and that, if you don't show up, you keep your original flight. Is that true?

It would be helpful if you shared whether you are a US Citizen or have Global Entry, and whether you have checked bags you need to wait for to re-check. That will impact the assessment of whether the ~2 hr connection is enough.

That said, AA's policy is to protect you in case of misconnect, which means that if you were confirmed on the earlier flight, and miss it, they'll re-accommodate you on the later (original) flight, although you risk your seat/cabin not being available at that time.

At normal airports and non-shutdown times 5 1/2 hours would be a walk in the park in MIA while at any time, let alone now, the 2:22 option done before departure is too close.

Arrive at MIA, go through the circus especially the security - if you don't have TSA Pre-check and you see connections security is backed up 10 miles, try exiting the secure area after customs, re-enter and check the E gates. E is a smaller terminal and is connected to D anyway. People with D boarding passes will queue at D even though you can use E thus often E is faster.

Once past the zoo if you enter at E try the E Admirals Club up the elevator as soon as you clear E security or if you clear security at D (or at connection security) try the D15 lounge or the new D30 combo club and see if they will change the flight.

Good recommendation. E's security is terribly slow and it's a solid 15 minute walk (each way) from D, so I'd only recommend doing this if the TSA line at D looks like ~100 people or more (give or take).

That is true, you won't lose your confirmed seat unless you get on the earlier flight.

That's correct.

Well, I was *going* to say that 2h+ was probably a safe bet at that time of day, but not *that* safe! :)

But your question now is delving into the realm of paid J passengers on standby for the flight vs. confirmed Y passengers on standby for an upgrade. Many gate agents prioritize the latter. I think if you confirm into Y, you'll have a good chance of staying at the top of the list as a paid J passenger (which I assume).
That said, there's currently 1 J seat left, if you are in paid J, you should be able to claim that, no? It's available in J/D/I inventories.
Alternatively, you could ask for a seat in row 17 which is normally crew rest on long flights and I believe has extra recline and a legrest. Not sure I'd want to get to SJU at 1:30am after flying in from Europe!

Was this an actual J (full fare business class, refundable and changeable at no charge), or a D or I fare? An AAnytime award (which is full fare J) or MilesAAver award (which isn't)? If FF J/AAnytime they should have given you the seat, D or I there is the fee for change thing and do you want to pay to change and/or reprice and thus you get lumped in a list with upgrades. That's what I suspect. Full fare J the Flagship Lounge would have handled it with aplomb I'd think.

1/2 an hour (in MIA with the shutdown, no less!) on the other hand is a terrific connection. Did you use connecting security, or did you exit baggage/customs and go back up to D or E?