Replace disks in ReadyNAS

You'll have to be more specific about the model numbers, OS versions, and RAID types to give a more specific answer.  In general, the answer is yes and you won't have to lose any data as long as you are running XRAID, but there are some limitations, especially if they are legacy machines still running OS4.2.x.  Since you said you don't care if there is data loss, then the answer is even more likely to be yes.

But you may want to re-think this unless the age of the drives is such that you just want to replace anyway.  Incrementally upgrading can help spread out when any future drive failures can occur.