No, the drive will still function as a spare.   You do need to upgrade the group firmware though, many improvements and fixes since then.  Especially, in how preemptive failures and drive faults are handled.  5.2.5 would be the minimum you would want to go to.  6.0.1 is the most current revision.   Best best, is to open a support case and let the engineers looks at the disks and see what's going on. 
Hi Guys, I am quite new to EqualLogic storage so need your advise. I have an array that has some disks with outdated firmware. What is the best way to update the firmware? Preferably without a downtime. Thanks in advance for your responses.
Upgrade firmware for a single disk