[CentOS] Failed boot drive

Unclear on how to recover from failed boot drive? SDA has failed but I'm still able to boot. I need to replace primary boot drive. Upon initial build I created a RAID 1 mirror of /boot dir/partition. How do I get system to boot off second or third drive so that I can replace /sda? I've successfully replaced /sdb & sdc raid 5 drives in the past but have always struggled to get primary boot drive replaced. Read HOWTO's in the past but it's been an unsuccessful pain and eventually just did a full rebuild.

I just read another HOWTO and as confused as I was several months ago, nothing on getting system to boot from alternate drive.