Re: USS Copernicus, NCC-640 or 623?
That sounds reasonable, and it reminds me of the issues with there being two apparent Melbournes fighting at W359: the Excelsior model which is quite visibly destroyed in "Emissary" and the already wrecked proto-Nebula model which Greg Jein (IIRC) built for the graveyard in TNG. The best suggestion I've seen regarding the name and registry issues is that the Nebula class Melbourne was a new build intended to replace the older Excelsior class ship, which presumably would have been decommissioned, and it was the Nebula version that was offered to Riker as a command. Then the Borg invaded and both ships were rushed to the battle and were casualties. It's a nice explanation.
"If you think you're brave enough to walk the path of honor, then follow me into the dragon's den."