What I think aborzenov is asking is, are your root volumes (vol0) on the disks in the internal slots of the 2050 itself, on the disks of the extra shelves or spread across the two.
If the internal disks contain any volume you want to keep things are going to be complex, especially if any volume is spread across internal and external disks. You don't really need to keep the root vol, as it will be completely different for a 3140 to a 2050.
If any volumes you want to keep is wholly on the external disks then you should be ok to just move the shelves across, assuming the 3140 already has some disks on its own which contain its own root vols.
Any other scenarios will involve a lot of work.
I assume you've the 3140 going spare rather than going to buy one new (it's an old model)