@edis, welcome to the Rockstor Community.
take a look here for using the btrfs-replace option:
https://rockstor.com/docs/data_loss.html#btrfs-replace
You can also that your feature request is already noted, but not implemented yet, due to the ongoing overhaul of Rockstor (moving off obsolete python versions, upgrading the django framework and the like, that’s driven by @phillxnet and @Flox).
We also had a recent thread around this (replace vs. add/remove), which you can find here:
I used the add/remove disk through the WebUI, but that took quite a bit of time, since I replaced all of my disks in the RAID pool.
either of these options should allow you to perform the actions without shutting down the NAS. In my opinion though, I would avoid major write operations during that time (e.g. backing up massive amounts of data to the NAS from other sources), as that will slow down the replacement process.