Yahoo makes it easy to export your contacts to CSV. I wanted to start with clean slate, so I exported them to CSV, opened them in Excel and edited them...when I wanted to import that back into Yahoo, guess what? NO, it does NOT support CSV import,even its own export!
So I had to go back to ever reliable Gmail - it imports Yahoo CSV
And Yahoo imports Gmail contacts!
Sounds convoluted? It indeed is...but that is the Google difference...