One can do the update and see how it goes; you will have some old configs hanging around for those things that have changed like moving an app’s location from one file tree to another, etc. You can always go back and do a clean install (if you have a separate /home partition) and things go wrong.
Those of us that have done this long enough have found that the “best” way is to have a separate /home partition and do a clean install. You should have all you emails, etc. still there and with very little config, you are back up with a clean install. Me, I image my /home partition; keep the old install; put the imaged /home on a new partition and do the clean install that way. The imaged /home serves as a backup too:)
The reason I ask is because I have headless web/file/multimedia server running in the basement, and to do an upgrade install I need to hook up a monitor up to it, spend an hour or two installing and setting things up…
You can update from one version to another, but as pointed out, you then have to sort out any config files that need to be fixed. You might also miss some new features that are in the new config files but not in the old ones, only an examination will reveal this. And if the new software is quite different (totally new version, or obsoleted by some better package), then you really have to figure out something.
On the other hand if you do a fresh install, you have to learn the new way of doing things from the start.
It all depends on which hassle you prefer.
Sometimes I do an update on a server that I have limited physical access to. I then get remote access working again if necessary and fix up the config files remotely.