Do not change to DTV it is a inferior tourist visa, compared to a real long term visa as the non-B, which have more benefits to the longterm stay in Thailand.
Remember you may have to give up having a Thai bankaccount if you change to DTV.
If choosing to apply for DTV, many requirements such as how long the 500.000 baht should have bern in the bank is Embassy dependant, so make sure you read the Embassy website thoroughly for those requirements.