Can I change my extension reason from retirement to marriage while on a 90-day O visa in Thailand?

There usually is no issue changing the reason for your extension AFTER you get the first extension for the reason you got the visa (so if you have a Non-O based on being over 50 you need to get that first year extension for that reason)

Sometimes the office will not let you do a 'reason change' (retirement to marriage) until the following year when your current extension is running out, but some offices will let you change the reason during the year extension.

Just know you lose all the time on the current extension and the new one starts the day you apply for it (because you're changing the reason for the extension it is not added to the expiration of the one you have now)