Swap drives from 2 drobo-fs to even out capacity

I have two drobo-fs. I want to turn one into a backup for the other. They are different sizes, so I want to swap one drive to the other to make them the same.
The drives are as follows (TB)
4 4
4 4
3 4
3 3
3 4
The left one is 24% free. The right one is 70% free.
I want to swap drive 3 from the left with drive 5 on the right so they are as follows:
4 4
4 4
4 4
3 3
3 3
Question: Should I format, wipe, or remove the partitions from both drives to avoid confusing the two before sliding a drive into each drobo that is from another drobo?