Aircraft approach category
I'm not sure if I've got the right sort of category here. Are we talking about 'aircraft approach category' which is part of the FAA definition to classify the airport? If not, and it is some instrument flight category, then please disregard the rest of this post.
If it is the FAA airport classification, then the Dash 8-300 is classified by the FAA (reference: AC150/5300-13 Airport Design) as A-III, which is aircraft approach category A.
The basis for their classification is the aircraft approach speed, and the limit for aircraft approach category A is <91 knots. The aircraft approach speed is defined as 1.3 times the stall speed in the landing configuration of aircraft at the max certified landing weight. I'm not sure what the stall speed of the Dash-8 is, but the FAA has the approach speed stated in their database as 90 knots. The Cat B limits are 91-<121 knots and Cat C is 121-<141 knots.
Even if the aircraft approach category was to change from A to B for some reason (while staying aircraft design group III), there would be no change in the airport standards (FAA Table 1-1 in AC150/5300-13). It is only moving from Cat B to Cat C (while staying aircraft design group III) that means a step up in terms of the runway strip (the FAA call this the runway safety area). The runway now has to be less undulating and have flatter gradients. However at 9 out of 10 airports, you’d be hard put to pick the difference. The 10th airport is probably one of those spectacular Dash-8 airports up the side of a mountain or similar.