Can I extend the new 45-day visa exempt entry stamp in Thailand?

If you are indeed from one of the 52 countries that get in on a visa exempt entry which is now 45 days there is no issue going to your immigration office and applying for a 30 day extension for 1900baht.

At most offices you can do it when you have 30 days or less left on your current stamp and they add the extension to your expiration date so you don't lose any time going early

Yes, 30 days extension.

Yes, 1900 baht, 30 days