When you remove a disk from the pool, part of the removal process is to move all the files off of the disks. You can skip "duplicated" files which may be a good idea!
Given that you don't have the space to remove the drives *and* maintain duplication... That would be the bigger issue here.
If you have the money, opt for the "advanced replacement". It requires a credit card (or debit card), and they put a hold on the card equal to the amount they charge for the HDD. Then they send one out to you immediately and give you a window of time to return the bad drive. Once they get it back, they remove the hold from your card. Otherwise, if you take too long, the charge goes through.
I'd recommend this method to you, because you can get the two new drives, add them to the pool also, and then remove the bad drives from the pool.
Otherwise, you'll want to use the "skip duplicate files" option when you remove the disks, and disable duplication temporarily. Which isn't optimal at all.
