Upgrading users is one of those "damned if you do, damned if you don't" scenarios. If you do: some users are unhappy about old hardware. If you don't: some users are unhappy about uptime and changing software.

Joyent's CEO was on HN a while back and mentioned that if users on the old FreeBSD boxes want to run on the new illumos boxes, they should send an email to support.