Can I switch from a Non OA Visa Retirement to a Digital Nomad Visa (DTV) while teaching online?

You need to let your current visa expire before you can apply for a new visa. You can not work on a retirement visa.

a 365-days Non-Imm-O/A Longstay Visa has a multi re-entry permit included. So, in its first year of validity, you cannot make it invalid, which is required before you can apply for another Thai visa-type. You could only wait for the expiry of the visa validity while you are outside of Thailand. In case you are NOT on the initial Non-O/A Visa anymore, but already on a 1-year extension of the stay permit, you just need to exit Thailand without a re-entry permit, this will invalidate the existing stay permit and you can apply for another visa . . . . .By the way, the Non-Imm-O/A Visa is NOT a "retirement visa". Actually it is a "Longstay Visa". The only TRUE retirement visa is the 90-days Non-Im-O Retirement Visa

You can't apply for a DTV while you hold a valid Non-OA visa, you can't legally work remotely from Thailand until you have a workation based DTV. Whether you can keep your Thai bank account will be down to your particular bank branch

You may have a bank account on a DTV but you can’t apply for one. You can’t work on retirement. If you wait until your Non O-A visa expires, you may apply for a DTV.