Having sat alongside civil airfield controllers in a combined radar room, while there are a number of differences, I'm not convinced that from an airfield perspective there's so much re-training to do that it could not be handled as OJT. That said, I think there is a significant amount of work to be done by SRG in terms of scoping the training and licensing requirement.
Area radar is a completely different kettle of fish and there are significant differences in the way in which business is conducted.