@Sue *********
it's difficult to open a bank account with a tourist visa so you might have trouble with that. You need the money in a Thai bank account already to apply for the conversion to non-O at immigration.
On the other hand many embassies require you to show 90 days of insurance to apply for the non-O overseas and you save yourself from that requirement applying inside the country.
Also if you already had a bank account, you have to prove the money in it came from overseas if you apply to convert inside the country but you don't need to prove that for the 1 year extension.