I'd agree with those above, who have said it is a totally different role. A depot driver has no experience of driving a train "in anger" - fog, adverse rail conditions, etc. The GE are atypical, in that the role is in your line of promotion. AFAIK, everywhere else, it is simply a cheap(er) option, for covering depot work.
The depot driver will still need front end turns, train handling, traction (unless already signed, to the same standard, and if moving company, they will probably retrain anyway) and part of a rules course, in which case it is probably easier to put them through a full one. Same as "off the street", and no different to what some TOCs have found, when they took on tamper drivers as "qualified." Plus they will still have to do the full PQA process, which is why most "qualified driver" ads, ask for 2+ years experience.
They should be a strong candidate for a trainee post, but qualified, a resounding "no".