I haven't found this to be the case.
You can safely edit everything about a given player while in the game, except his name (and number*), since there are many instances of players sharing a single name slots. In these cases, if you change the name, it will change it for all players linked to the name. Better to do name work in the editor.
*As for the numbers, if you change a player's number in the game, it will almost always change their play-by-play (PBP) from the PBP assigned to the number. Exceptions to this include the few players still around from the original (e.g., Tom Brady) and a few other players whose full names can be used from the original. Now if he player doesn't have a PBP name in the game anyway (e.g., Garoppolo), it doesn't matter, so go ahead and change his number in game.
You can safely change all other attributes, including ratings. **Just be sure to back out of the edit screens using the B button, and NOT the start button.**
Last thing: dreads can only be toggled on/off in the editor.