Will an expired passport affect my DTV visa application in Thailand?

I had the same thing. When I renewed my passport the local immigration office transferred my visa to the new passport for a small fee and recommended I always travel with my old passport too as the passport number on the DTV printout will never be updated and will always have my old passport number on it. From my experience, this means that when they scan your new passport at the border it won’t pop up to say you have DTV, so you need to be ready to tell them you have a DTV, show them the printout, and show them the stamps to prove that your visa was transferred every time you enter the country :)

I mean it is probably better to get a new passport but usually 6 months is enough to get the visa. But you will need to move the visa to the new passport when the time comes. That happens in the local immigration office.