Yes, you can put the tix on Hold at AA.com. Write down the record locator and then call AA, give them the record locator and tell them you want to upgrade. Keep in on Hold status and then go back to AA.com to Purchase. I do it all the time.

Also, as one who goes NYC to Hawaii many times, you should consider flying out of LGA to ORD or DFW and back instead of JFK through LAX I presume. A much more enjoyable flight in the 763s out of ORD or DFW on an 8 or 9 hour flight and you get a chance at the new biz class seats.

BTW, the assumption is you already know before hand what flights are upgradeable (A inventory) before you book the coach tix. If you are looking for an upgrade.