One of my clients upgraded the OS on its Apple Xserve G5 the other day. 10.4 is much nicer than 10.3, and generally speaking the update went pretty well.
It's a bit odd, though, that the updater managed to make the server forget a few things. Like how to route incoming mail addressed to [email protected] Or that it was a DNS server for the .local domain.
Fortunately, fixing the issue was simply a case of recovering the /etc/aliases file from the backup, and reminding the DNS what domains it was in charge of (the zone files themselves were unharmed - it just decided not to refer to them!). And the more esoteric stuff (like some bespoke CTI software I wrote for them) carried on working without missing a beat.
It's just a bit weird that some config files got blown away ... hope there aren't some more that we've not spotted yet!