Changing from comfortable hydronic heating system to scorch air system is one of the biggest regret people made.
The key word being "comfortable". It sounds like this botched attempt at a conversion to pumped hydronic doesn't exactly fill that bill, and unless there's decent help to be had in fixing it...
Done
right hot air doesn't have to be terrible (but most don't fit the definition of "right" either.)
If the thing is working well enough to get by, it's worth continuing the talent search for competent hydronic designer. It isn't necessarily very expensive to fix, but it takes some degree of competence to unravel the hack and fix it, rather than layering on more hacks. It's still worth doing the room by room Manual-J and the radiation analysis yourself. If you're uncomfortable doing the load numbers yourself, hire a P.E. or RESNET rater or some competent energy nerd to run the Manual-J, letting them know you want it to be as aggressive as defensively possible. You can bet the average hot-air hack won't run those numbers, leaving you with an oversized and unlikely unbalanced hot air system for your troubles. (Even those HVAC contractors who bother to run Manual-Js have a propensity to being ridiculously conservative, oversizing by more than 1.5x on the real loads, then using a multiplier on that already oversized number.)
The AC system may or may not have been done well, and even if the rooms are reasonably temperature balanced with the AC, the cooling loads aren't often sufficiently proportional to the heating loads to deliver decent heating season temperature balance using the same ducts & flow volumes.
If any of the ducts & air handler are in an attic above the insulation, outside the pressure & thermal boundary of the house the efficiency takes a big hit in both modes.
If the supplies & returns aren't flow-balanced well it will also drive outdoor air infiltration, resulting in excessively dry wintertime air.
All of that would have to be assessed before deciding to mothball the botched hydronic system rather than fixing it.
Full basement?
How many floors?
Window types?
Insulation levels?