1. I assume BKK-ICN is operated by KE. Are you in their version of premium economy? Check the rules for using GUCs on KE regarding fare class requirements and whether codeshares are OK. I'd be surprised if a GUC can be used to get Prestige class in KE on a DL award ticket.

2. If there's been a schedule change or you have an illegal connection, DL can open award space on any DL operated flight(s), but can't force KE to open award inventory for you. You might be able to reroute to a later ICN departure to another DL hub that has a same day connection to LGA.

3. Remember that if you use the GUC, it also upgrades into domestic FC if a FC seat is available, with the inventory bucket restrictions. Also PS fares book into C+ on domestic connecting flights.

4. Another possibility might be to change to a ICN-JFK nonstop.