Yes this should be possible, but it may take some time. And if you are on an earlier version of Rockstor it may end up timing out on the Web-UI. But it should still complete in the background so don’t be tempted to do anything else if you encounter the error seen in this now closed issue:
which was included in 3.9.2-49 in Octobet 2019
And our newer Rockstor 4 (release candidate) variant also has this fix. The installer for which is still in DIY stage here:
But as always it’s best to do a backup first if you care about the data as this process is very intense on the filesystem and will effectively re-write most of the data stored on the pool. This is required as each chunk of data is being re-written in the new profile (raid level), this also explains the length of time this can take.
Just to add to @phillxnet words, I have done this very thing on Rockstor 4 several times in test, and once on my ‘production’ box - always successfully. This is one thing with BTRFS I really like although I won’t need to do it often.
Basically when you ‘re-RAID’ from 1 to single, BTRFS passes the data through its allocator, back on to the disks - that’s the balance thing happening. Depending on how much data you have, this can take a while (eg hours). The status of the balance can be seen on the detail page for the pool. The beauty is you can continue using the NAS while the balance is underway if needed as the process happens with the device still online.
As ever, make sure you have a backup beforehand - this should always be your first step.