"Only" a Dash? Having flown both types, my experience says that switching from a F100 to a Dash 8 is much more difficult than going to the Fokker with previous Dash 8 experience.
The Dash is only a little bit slower than the Fokker but much more complicated system-wise and noticeably less forgiving to handle. The Fokker on the other hand may not be state of the art any more, but it is very well thought out and has only very few pitfalls that need to be minded.
So a reasonably competent Dash flyer should be able to transition to the Fokker without troubles, as has been demonstrated dozens of times in my company. Giving Dash copilots an upgrade to the Fokker L/H seat has been standard around here for years and never been a problem.