Going from dapper to hardy? Hmmm…it should be OK, but you’ll need to plan for some additional downtime (on top of the upgrade time) to insure all of the Virtualmin-provided packages get updated, as well. You’ll probably need to update the Virtualmin source manually for that to point to the hardy repo rather than dappy.
Anyway, both OS versions are supported by both Webmin and Virtualmin, and we provide packages for both. So, the differences should be relatively minor, and problems will be small. Let us know if any issues show up, and we’ll help you get them resolved (but again, plan for things to break–OS upgrades pretty much always break stuff, and even if Virtualmin stuff goes completely without a hitch, I kinda doubt the OS upgrade will).
I was under the impression (though I don’t know exactly why, I seem to recall reading it somewhere) that the upgrade between LTS versions was extensively tested, since this is always the version in use in mission critical environments. So, I’m guessing it’ll be safer to go from LTS to LTS than to go from LTS to non-LTS to non-LTS to non-LTS to LTS. But, what do I know? I’ve not done a lot of Ubuntu upgrades.
Sweet! Sounds like Ubuntu did pretty well on the upgrade process (Debian is also historically really good on this front, better than CentOS/RHEL, for sure).