I'd reckon they'll hang onto the 42-320's for as long as possible. To change to a 42-500 you will increase the lease cost, increase landing fees and increase fuel costs by about 10%. I guess it would depend if the maintenance costs outweigh these costs.
The 42-600's would cost too much to be worthwhile putting on routes the likes of KIR and BLK even if there is a reduced fuel burn.
I do see the need for Stobart to keep a few of these smaller aircraft. They provide the cheapest possible way to start a new route and can also make marginal routes profitable