This is the first league offering the ability to rename your draft picks - when you look at the player card, there will be a "rename" button available. You can change a player's name up through the training camp stage as long as the name is unique to the league and is available in the names database.

I got a note that said I have to buy credits to rename a player?

That's true - you have to have a minimum of 6 credits in your account to do any of the customization features, but the free credits cap out at 5.

do the customizations cost credits or do I just need them in my account?

It costs 1 credit to rename a player. The custom league ranges from 3 to 7 credits, depending on what you are changing (it costs more to move a team, less to rename them)