My understanding is that the Tornado-G uses new trucks but old launchers. Same goes with the Grad variants that were modernized in Eastern Europe and now handed to Ukraine. It is understandable because you're dealing with old vintage trucks, the condition of whom are questionable in the battlefield. Some may already be corroded beyond restoration. For situations like these, it's better to salvage the rocket launchers and marry them with a new truck. The MoD has new requirements for fragment resistant cabins, which have resisted fragments from 155mm munition and even HIMARS.
There are also those from the depots that are still restorable and can run. They are likely brought to the battlefield out of expediency.
Ural-4320 truck has been around for a long time. They started producing those in 1977 and even today. So they can always attach a new 4320 truck to an old launcher. Of course they would be many evolutions and variants over the years, that the 4320 can be described more like a family including cab forward arrangement. These trucks are used for all purposes including civilian.
It would be easier for an old Grad system to exchange the old truck to a new one, reuse the MLRS, the flutes or pipe organ whatever you want to call it, then scrap the old chassis or cannibalize it for parts.
If the truck breaks they can switch the MLRS to a new chassis, while they repair, even refit or extensively modernize the old chassis. Or it can be cannibalized and join the pool or parts. The flutes can return back to service as soon as possible.
The cost of maintaining an old truck is much more expensive than a new one. Downtime is also bad, because this is a critical mission situation. The chassis itself can be treated on a more disposable manner than say an armored tracked vehicle.