wow ... the last ... so you come 15 days to early .. you had only 75 days and then for the next time 90 days again. So if you come every time 15 days early you have to go a little bit more often then 4 x a year. If you come one week later you not "win" one week. They take the date you should have come.
I arrived at Suvanabhum and did show 2 passports. (the old and the new) Did get the normal stamp for the visa based on retirement up to the date the old visa in the old passport mentioned. So arrived in july and the old visa was up to december Suvanabhum stamped a visa up to december in my new passport and i had to go to my regular immigration office in Bueng Kan, who did the transferring of the old first original visum in to my new passport at no costs.
Had this 2 times in Bueng Kn immigration. No problem. Just have your new and your old passport (with the old original visa s) at hand and they took over the old first "retirement" visa in to the new passport. So got a nw first retirement visa with old the old dates in my new passport. No letters from the embassy needed. Just always ask your vernment who gives out the new passport to not pt a whole in the pages with he visas in the old passport.