the people who said they were on ED visa but got another visa outside of Thailand were not on a visa at all but on an extension of stay. Those on valid visas (such as the METV) have been refused visas by Thai consulates and told to wait until the visa expired, the same will be true for the DTV. No way to get another visa from a consulate so it will be essential that the company they wish to work for supplies the paperwork to convert the visas at an immigration office inside Thailand. I can't see any way out of that.
They are not banks so the answer is maybe, it will depend on whether the embassy you apply via will accept them. There was a report yesterday of Thai immigration refusing to accept Wise for an extension to the DTV
Visas generally can't be cancelled, you either use them or not. As the DTV is based on tourism it is possible to change it to another type of visa inside Thailand at an immigration office, just as you can for a normal tourist visa or visa exempt entry.