So, it's indeed possible to replace the disks of a bootable RAID-1 volume on a RocketRaid 1820A card. The box in question is running Windows 2003 server. It has hot-swap SATA drive bays. This makes it easy- if you don't have bays it becomes a lot more challenging. Before I go on, of course, make sure you back your stuff up. I didn't, but you should. I'm not responsible for your data, job, etc...

So, here we go:

That's it! I did this to replace a 40GB RAID-1 that was getting ancient with some newer 200GB disks, and it worked great for me.

Hope this was helpful!