If you have a 1 year extension for Thai wife and divorce her. Can you just go to the immigration office and change your extension to a 1 year extension for 50+ if you have the money? Or do you need to start again with a non O visa first?

You can go to immigration with the divorce decree to have your existing extension canceled and apply for one based upon retirement.

Can you apply even if there is 4-5 months left on the old extension? And without leaving the country?

11 minutes ago, bangkoklasse said:

Can you apply even if there is 4-5 months left on the old extension? And without leaving the country?

As I wrote immigration will cancel your current extension and you will apply for a new one year extension based upon retirement.

No need to leave the country.

Thank You

Just to be clear your extension validity ends with the divorce - you have no option to wait.  You must immediately change extension or leave.