The warranty on your Honda PCX 150 is issued by APHonda (the Thai Honda motorcycle distributor) and with that in mind all official Honda motorcycle dealers should honor a warranty claim.
It's however understandable that a dealer doesn't like to provide warranty on a motorcycle or scooter he not sold. Maybe with the help of the APHonda Call Center you can move your service from your old dealer to the new dealer. You can call the Honda Call Center at 02-725 4000 (be patient it takes sometimes a bit of time before you get a English speaking person)
I own a 2015 PCX 150 and a 2015 Scoopy I. I took then to Chiang Mai Honda this morning to an oil change on both bikes (4,000km change) and airfilter replacement. I also had them repair the lock for the seat on the PCX. The bike has 4,400km and is 4 or 5 months old. I was told that if I wanted to have the lock repaired under the warranty I would have to return to the place of purchase. All my documents are in Thai so I can't read them but I did print the Users manual from the internet in English. It that it said any authorized Honda Dealer would honor my warranty. So, which is correct? It was a small amount of money but is something more serious occurs I need to know what to do. Please feel free to contact me at any time.
Looking forward to hearing from you and thanks in advance for your assistance.