I opened a project which I had been working on in v6 and got a huge number of diagnostics in ERC. Actually I hadn’t done anything substantial with the project since v5, it’s a back burner project which I haven’t finished, still tinkering. Here’s how I dealt with the list:
GND and +5V symbols changed in library: Update symbols from library. I had to save the schematic before running the ERC again.
Lots of elements off grid: Not sure how this happened, maybe it’s a hangover from v5. Selected everything with Ctrl-A then Right Click > Align elements to grid
Female connectors couldn’t be found: Went to Tools > Edit Symbol Fields, selected those connectors one at a time and changed to socket connectors. (There was a topic about why no more sex for connectors.)
A couple of hidden power pins on one IC caused conflict in net names. Fortunately it was my symbol (not sure why I made the pins hidden, maybe I copied from another symbol), edited those pins making them vislble.
So now I’m left with a couple of units not used warnings which are fair enough.