This is how that change in entry stamp time worked out and it starts Oct 1st runs until Mar 31st.
As you can see ONLY the countries that get the 30 day visa EXEMPT entry are eligible for the change from 30 day to 45 day entry on Oct 1st until Mar 31st and they can get one 30 day extension on that entry stamp at the immigration office for 1900baht.
The people from the six countries that get a 30 day visa WAIVER entry are NOT part of the program and will continue to get 30 days and it cannot be extended. Those countries are Lao, Macao, Hong Kong, Mongolia Vietnam & Russia.
Also people from the 17 countries who show up here and BUY a 15 day visa on arrival for 2000baht will get 30 days visa on arrival BUT it will still cost 2000baht (They list 18 countries for VOA on that list but Saudiz get in on visa exempt now, NOT voa so it's 17)
TLDR : Answer Summary
Starting October 1st until March 31st, travelers from specific countries eligible for a 30-day visa exempt entry will now receive a 45-day entry stamp instead. They can also apply for a one-time 30-day extension at the immigration office for 1,900 baht. However, travelers from six countries that receive a 30-day visa waiver will not benefit from this change and will continue to receive 30 days without the option for extension. Additionally, travelers from 17 countries purchasing a 15-day visa on arrival for 2,000 baht will now get 30 days on arrival, while still paying the same fee.