since comfort has always been an issue, and is not a new thing,
I'd want to be sure that the ducts are delivering cfm of air required to
each room (this is based on size of room/orientation/windows etc..part of
a room by room load calc..not whole house quick calc).
I'd look for someone to measure air flow & verify what is actually
being delivered. easiest way I know of to measure air flow to each
room is with a flow hood.
replacing system without addressing air flow issues isn't going
to solve the comfort issue.
also, making sure ducts & returns are mastic sealed would be
a part of any job.
best of luck.
The cure of the part should not be attempted without the cure of the whole. ~Plato