With older gear, it seems the best policy is don't mess with it.
When does stuff fail? One of the most frequent failure scenarios is when it's powered up.
So a drive can run for years and you lose power, and it doesn't spin back up.
We just went through this a year ago when a major customer lost power to their downtown data center due to a water main break and the water coming into the power switch panel through the conduit that was feeding one of the power sources.
So all the automated power switching, batteries, etc were no good because the gear flooded.
Anyway, we had 3 or 4 days of 24 hours on site getting equipment up and running again as drives failed to spin up after being powered off.
Systems that ran since 1997 were losing drives and power supplies.
So stuff will run, until something happens.
I see the same sort of thing when people want to upgrade older gear. It's nerve racking to power it off because one doesn't know if it will power back on, or we create more problems, etc.
We do sanity power cycles and reboots on major service actions just to make sure the system will reboot cleanly before we make any changes.
So I'd add to the plan to add a USB or some sort of drive before you crack the case, because things may not appear.
Also, I really doubt you can remove one drive and mirror that drive to a new drive. I think you would have to add the two new drives, mirror them and then copy the data to the new drives.
But check with the RAID controller docs to see what can be done.