Short answer, copy the files elsewhere, format to XFS or BTRFS, copy files back.
There is NO way to switch file systems in Unraid without losing the data that is on the drive.
Take a look at the sticky thread in the general support area for some strategies, my favorite is either a new additional drive as large or larger than any existing data drive, or if you have enough cumulative free space to empty your largest data drive by copying files to other drives.
Either way, copying is much faster than moving for a bunch of reasons, plus you can verify the copy is good before formatting the ReiserFS source drive.
It's possible you could get away with copying the data from the read only disk2 to another disk if you have enough space, then reformat that disk after the data on that disk is safely copied.