Dale's right, the 400 just wouldn't work out of PLH, and the additional running costs must be unattractive at the moment.
Learjet, I may be wrong, but I think you can technically get away with a differences course moving from the 300 to the 400, although I know that Flybe went down the full course for their transitioning crews. Bombardier considered changing many things such as the overhead panel design and the crew alerting system (light box) to more modern designs, but went with versions of the old items to facilitate a common type.
Another 300 would have been great, but the Q200 is an interesting choice and may prove useful (and efficient) for charter work and developing new routes in the future, as well as occassional tech cover. Cat III would be a bonus given all the weather global warming seems to be bringing our way....