Can I convert my ED visa to an O visa in Thailand if I'm over 50 and have the required funds?

If you are on an actual ED visa you can't convert at all. But if you are on an extension of stay it might be possible to change the reason for your extension directly to an extension based on retirement without getting a Non-O visa at all. Have a word with your immigration office and see if they will allow it

the only downside could be, your ED visa could be frowned upon when you enter visa-exempt, because Immigration knows it is the most misused visaclass. So you should be prepared to show 20,000 THB in cash, a few booked nights, and best is to have an onward travel proof (a cheap one way ticket out of Thailand within 60 days, just in case). And if questioned, tell them what you intend to do

you need to exit without a re-entry permit for your ED-stay permit, this will invalidate it. Then re-enter visa-exempt, visit Immigration and apply for the "change of visa type" from the exempt entry to the initial 90-days Non-Imm-O Retirement Visa. Application fee is 2000 Baht. You need a rental contract and proof that the 800,000 THB are already sitting in your Thai bank account, and a minimum of 15 days left on your entry stamp. As soon as the money has sat in the account for 2 months, you can apply for the 1-year Extension of the Stay Permit based on Retirement , the application fee is 1900 Baht