If A checks are going to change, then B-checks should probably change, too.
It's only a minor added complication, as long as it's obvious how long checks will take.
But it's also a negligible benefit. The current 5 & 24 hour times are arbitrary. If it changed to 3 & 12, 4 & 18, 5 & 24, 6 & 36 for the 4 sizes of aircraft, it'd still be completely arbitrary, and the extra profits generated by small/medium planes will be minor.
It's not a bad change to make, but it should be right down the bottom of the priorities list.