Is my DTV visa still valid if I switch to a non-B visa?

You mis understand how a visa works.

You can only hold 1 visa at the time.

The DTV does NOT allow you to work inside Thailand, which simply means if you keep your DTV and start working inside Thailand you are an illegal person working illegal with a lots of consequenses when something happens.

You must cancel this DTV and apply for a NEW Non Immigrant B visa, this could be done at a local Immigration Office when you and your employer can meet all requirements.

Most of the time your employer will tell you to apply abroad.

THe Non Immigrant B visa gives you 90 days on entry, than you pickup your work permit and can start working. Before these 90 days ends, you apply at local Immirgation Office for an Extension of Stay based on employment.

In case you quit working, you get fired YOU have to cancel your work permit AND your Extension of Stay using the last day of work.

You may apply for a NEW extension of stay based on another reason, if you can meet the requirements you get the extension, if you can NOT meet the requirements you get a stamp non approved and 7 days to leave the country.

You can than apply for a new visa to visit Thailand.

You can't have two visas at once.

Particularly for Non–B Visa/E–Visa as it is an extensive amount of paperwork, a Thai Embassy/Thai Consulate outside Thailand and, especially, a local Thai Immigration Office inside Thailand will definitely tell you to either:

go figure it out yourself and find a Thai gov't entity to cancel your active DTV (most likely the Thai Embassy/Thai Consulate that issued your current DTV Visa/E–Visa) before applying for a Non–B Visa/E–Visa with them

or

they will cancel your active DTV as part of issuing a Non–B Visa/E–Visa

don't see any other pathways from DTV to Non–B

Where did you get the Non-B visa? A DTV can be converted to another visa type inside Thailand but the DTV is then gone. If you got the visa outside of Thailand you should have had to cancel the DTV

Cant hold 2 visa's

Legally you should have had the DTV cancelled before they issued the non imm B.

Did they ? Did you leave apply and re-enter ?? I believed you couldnt go from a DTV to a non imm B incountry but BOI seems to steamroller a lot of 'established rules' (was it a BOI application for the non imm) ??

You can't have two valid visas at the time. So now, either your DTV is already invalid or your non-B is already invalid. Which one did you get first?