Honestly, I don’t think that’s a good idea. Swapping out the drives one by one means you’re basically triggering a rebuild every time you add a new disk. From a risk standpoint, that leaves your system unprotected for a long time, and performance will take a big hit the entire time too…
If it were me, I’d rather just back everything up, format the system, set up the new RAID from scratch, and then copy the data back. And you’ll be up and running more cleanly that way.