Are you actually planning on going for the 2nd 5 year? I read a report from someone who went to immigration with everything that should be required, including the 3 million in a Thai bank account, and they were flat out refused by immigration and told it's not possible.
I believe Tod Daniels said most people abandon the 2nd 5 years and go and get a new non-OX visa.
If you leave with a re-entry permit, you can be gone for any amount of time as long as you're back in time to apply for your next extension.
You cannot "convert" to a DTV. Tourist visas are only issued by Thai embassies outside of Thailand. It wouldn't make sense to be able to get a tourist visa to a country while you're already there.
Did you actually come here on a TOURIST VISA, like your post says? A tourist visa is something you would have had to apply for and pay for through the Thai e-visa system before you traveled to Thailand. If you actually have a tourist visa, then you go to Chaeng Watthana.
If you arrived without any visa at all, and you entered visa exempt (free 60-day entry stamp) then you would go to IT Square Laksi.