I’ve done this. You might consider waiting till near the end of your current extension. The new year starts the day it is changed, not tacked on to the current one.
I don’t remember those forms, I switched to retirement extension a couple years ago. But in the past Korat immigration has been very responsive to email. (In English)
I’ve never had to do that at Korat. No form, no copies. Just hand them the passport, 2 minutes later they hand it back with the new paper with your next date.