While it will be double the work, I would suggest staying away from a new OS until you (and the community) had a chance to figure it all out. Also, see here: http://www.apple.com/osx/whats-new/features.html that Apple has announced that Mountain will contain Xsan 3. So there is yet another wrinkle in your plan. There is next to no public details about this version.
Snow (2.2.x) to Lion (2.3) should be a relatively easy migration. However, I suggest some of the obvious:
1: Have a confirmed backup
2: Make sure you estimate the time properly so as to not interrupt workflow - calculate for the worst
3: Remove all your clients from the SAN and upgrade them to Lion independently of SAN interaction
4: Stop the volume, validate it with cvfsck
5: Consider running the update on both controllers concurrently or remove the backup from the san so there is no interference.
Now, if you have the luxury, a nuke and pave often makes the most sense as you can ensure that the system is the cleanest. It can also reset the san volume and eliminate any pent up fragmentation.
Review these Kbase articles:
Thanks for the input. I'll read the KBs. It is always good to restate the obvious, though I had most of that in mind. Don't think we can have the down time to do clean install. Backup is issue also with 32 TB ( 2 x 16TB chassis) and no LTO library. I hadn't seen the Mountain Lion XSAN 3 listing. Adds another unknown factor.
By the way, the boys at Promise will send you loaner hardware. You might want to give them a call. This will allow you to offload everything to a direct attached array so you have some level of backup.
Ya, that Xsan 3 move is going to be interesting. There are no publicly disclosed features so planning for the move remains a mystery to most.
Good luck. Make sure you backup the config folder so you have a fallback.