Is there a step by step procedure available on upgrading Cisco IOS on switches running HSRP? Also, what kind of impact will moving from 12.2 to 15.2 have on HSRP and the IOS version operability overall?
Personally I would do the standby first and then the active just in case the upgrade fails although it's not going to make a huge difference either way.
As for new IOS I doubt it will affect HSRP as it has been around a long time and it a stable feature.
Obviously you need to check your preferred IOS mode any relevant bugs.
For HSRP, if you have priority and preemption enabled, you can upgrade the stand-by switch first and reboot it. There should be no impact since that is the stand-by switch. After that, you can upgrade the active and reboot. Once you reboot the active switch the stand-by should take over and when the active switch is back online it will take over again. As far as impact between versions read the release notes for the version you want to upgrade to so you are aware of changes, bugs, caveat, etc.. I would do the upgrade in a maintenance window just in case something goes wrong.
Before you upgrade the active gateway, I would suggest you have priorities and preempt set and by changing the priority, have the gateway migrate before you reboot the active. This to avoid the blackhole while the standby detects the lost of the active gateway.
15.2 should support whatever HSRP you're running on 12.2. Once you move to 15.2, it might offer some new HSRP features, like HSRPv2 or tie in with BFD (I don't recall what IOS versions offered what new HSRP feature).
Thanks. From what I see, the priorities are at their defaults (100) but preempt is enabled for all the vlan Virtual IPs. What should the Priority be set to, or, what is the process of manually failing over to the standby unit (with as little downtime as possible) while the active reboots, and then failing back?
Also, I checked the flash space and there is only about 21MB remaining on each unit with only the critical files (which includes the current IOS at 18MB), while the new bin file needed for 15.2 is 24MB. The release notes show this HW is supported, but how do I upgrade or roll back if both can't fit in flash storage?
All you need to do is have the inactive/stand-by gateway have a higher priority, which can be accomplished by setting it higher or setting the active gateway's priority lower. Depending on your hello timers, the gateway will migrate based on those and it should be relatively hitless. ("relatively" becuase, assuming you're using a switch, the switch has to change the destination port, in its MAC table.)
As to having insufficient space, often you can delete the active running image from flash to provide space to load the replacement. You then hope your device doesn't take a power hit during the time you're loading the new image. If it does, you might be able to relatively quickly recover by using a TFTP server on the same network or use a USB/CF slot, that some devices have, from which you can load (perhaps even boot) from.