ReadyNAS Ultra 6 Plus

I suggest you look at the WDC Red Pro series. The largest size you should use with your NAS is 4 TB, unless you are prepared to switch to OS6.  5x4TB would give you a 16 TB volume size, which is about the max for your firmware.  You could also add a 6th disk for dual-redundancy (RAID-6) - though that will hurt write performance some.

If you are replacing them all, then one option is to back up the data, install all the new disks at once, and power up.  That will do a fresh factory install.  Then reconfigure, reinstall any add-add-ons, and recover the data from backup.  You might consider switching to OS6 if you are taking this path.

The other approach is to replace one disk at a time.  You do that with the NAS running - waiting for each disk to fully resync before starting the next.  You won't see any volume increase until the 2nd disk is resynced.  One nuance - there is an 8 TiB limit to volume growth over its lifetime.  If your initial install was 5x2TB, then you won't hit this.  But if your initial install was 1x2TB (and you then added the disks one at a time), then you are already near this limit.  If you do hit it, then the factory reset approach is your best option,

One other disadvantage is that the one-at-a-time approach does stress the disks - every sector of every disk is either read or written every resync.  If disks are marginal, then this can trigger multiple disk failures.  I think the odds are low in your case, but that can/does happen sometimes.

FWIW, one reallocated sector isn't that concerning.  Though sometimes they build very quickly.  It would be a good idea to run regular volume scrubs (and put a backup plan in place if you don't have one right now).