Well duh... I should look at these images at full size rather than glancing at the thumbnails.
The Grad is being replaced by a Tornado-G model, but AFAIK it looks more or less the same as a normal Grad. So maybe its just a modernization, while the 122mm pallets will be used on similar size but new trucks tha might not be ready yet; which explains why we've only seen Uragan and Smersh sized trucks
That is possible, but I suspect that the future plan is for the Grad lookalike Tornado-G to just replace the Grad vehicles... they seem to have fixed tubes rather than loaded pallets.
TO me therefore I suspect the other vehicle we see with the separate pallets that seems to be a replacement for both the Uragan and Smerch that can carry two pallets so assuming 122mm, 220mm and 300mm pallets in any combination should be a useful flexible vehicle.
On other vehicle we have seen is a light truck with a single pallet... now assuming the single pallet light truck uses standard pallets that means it could carry one 122mm, 220mm 0r 300mm pallet to support different units over different ranges of targets.
This would mean for light units that use light rockets (122mm) they could use Tornado-G, while light mobile units might use the light truck Tornado with a single pallet capacity, while heavier units or units than need heavy fire power could use the other Tornado with the two pallets.