Hello phisherpryce, You can replace the disk as it is in a bypassed stated and powered down.  When you replace the drive if you are still getting the bypassed error then we will need to get a support bundle from your MD so that we can look at the errors & see what is causing the drive to be bypassed.
We have an MD3200i configured in RAID6 with 12 disks, no hot spares. The disk in slot 1 was showing as failed and the disk is slot 2 was showing as bypassed with SAA: No.  We replaced the disk in slot 1 and the rebuild completed successfully, the disk in slot 1 is now optimal.  The disk in slot 2 is still showing as bypassed with SAA: No and the array is still showing as degraded.  Can we safely replace the bypassed disk in slot 2 even though SAA is No?
MD3200i Disk showing as bypassed and SAA: No