One big factor in the success or failure of a DEC is aircraft type. If a person has been flying one type of aircraft for several years for company A, then moves to company B to upgrade on the same plane, then I would say the success chance is very high as all they really have to do is adapt to the new company's procedures. It is more difficult (although many people have done it successfully) to move from one type of airplane as an FO and upgrade on a completely different type. Probably the most difficult is moving from prop to jet or vice versa. In that case I'd say it'd be highly beneficial to get some right seat time first just to learn the new plane.

Guys at mainline do this all the time. You can be right seat of an airbus for years then upgrade to the -88 or 737 or some other completely different airplane and have no problems. Granted the company procedures will all be the same, just various nuances for different aircraft types.