I have the same issue. Long time player and prefer to sign up wiht my usual team names etc. not Cool Team. Pretty lame. Disappointed in Yahoo on this. This will create a situation when it might be possible to have two teams wanting the same name after being assigned to a league. Yahoo needs to revisit this change ASAP!
You must be signed up for a Public league, still waiting for league assignment, right? Yahoo did away with the name your team step for Public leagues. You'll have to wait until your team gets drafted/assigned to a league. Then you will be able to edit team name, uniform, upload a pic, etc.