I recently updated to Ubuntu 18.04 and I made the mistake of upgrading from an older 17.04 install. I upgraded to 17.10 and then up to 18.04. I compiled my wireless card drivers. I rolled up my cat5 cable.
What I didn’t realize until I was about to get started in with my usual customization is that I had originally installed the 32bit version of the operating system back a year or two ago. Many packages and programs are moving away from supporting 32bit by default, which caused problems when I was trying to install other software.
So I grabbed my 18.04 image boot USB drive, and started over. Everything went much smoother this time.
So the lesson here is that when you see the recommendation to start with a fresh install rather than upgrade up two major releases, there is a reason for it.