Can't see why you can't just physically swap them over, my understanding is that the warranty is on the heads and shelves. You own the disks so can choose to install them in whatever netapp devices you see fit.
Yes it is supported... you can't hot remove though but you can hot add.. so the 3240 needs to be shutdown (even if just unassigned spares) to have a supported way to remove the shelf...then you can hot add to the 2040. The raid labels are different between 8 and 7.3 but since not in an aggregate and unassigned you should not have an issue.
For support...you already pay for that on the controllers and shelves...definitely run it by your netapp rep but on renewal you will pay for the changed shelf count on each node.