I recently had occasion to use some BXP1 upgrades on JFK-LAX-HNL-(AA*/HA)-OGG-LAX-JFK and wanted to share some important notes:

1) Avoid using upgrade certs on itineraries that include codeshare flights operated by another carrier.

The AA codeshare segment operated by HA seemed to cause all sorts of problems with our itinerary. First, it took us 20 minutes and 2 agents working together at the JFK Flagship counter to check in for our outbound because of something about the way the reservation was coded behind the scenes.

Second, we were almost denied boarding to our HA flight. Fortunately, I noticed that our AA-issued boarding pass said "Not Valid without Flight Coupon" (which we were not given at JFK) and decided I should check at the desk in OGG - agent did lots and lots of typing and tried to call AA, saying something about HA being "unable to take control". Eventually, the HA supervisor just told us to board and said they would audit the ticket later. We lost our seats, were the last to board, had the gate agent come aboard to check our tickets again, and had to submit a retro credit request for the 250 AA miles. Not a great way to spend your 3rd segment of the day, but we were on our way to OGG at least.

2) In the event of OSO, cleared BXP1 upgrades cannot be transferred directly to rebooked flights unless upgrade inventory is available.

Unlike eVIPs/SWUs and Award Upgrades that get reticketed as soon as the upgrade clears, BXP1 upgrades to not get reticketed until you show up at the airport (or ticket office) and surrender the physical certificates. So, if your flight is cancelled (which happened on our return), you will be protected in Coach (not F/J as may happen with the other upgrade instruments) and the agents have no discretion about transferring your already-cleared upgrade to another flight unless the appropriate inventory (A,C) is available.

With all that said, we arrived at OGG to check in for our rebooked OGG-DFW-EWR itinerary with much trepidation because we'd left our hotel with A2 inventory showing for OGG-DFW but were unable to clear by phone because we'd already checked in (unchecking us would apparently boot us from our DFW-EWR coach seats due to the long waitlist) and expected confusion over the certs at this small station. Nothing could have been further from the truth - the counter agents were familiar with the certs and 10 minutes later, we walked away with our F boarding passes (with the lie-flat seats much appreciated for our long redeye). ^