If you want to get top players without losing any of the valuable players in your squad then perform the following process. With the salary cap off, find a player you want (for example, Calvin Johnson). Select him, go to 'Edit Player', and change his status to a QB. When you do this his rating will drop to 12. Then press 'Start' and choose to keep the player without saving. Now when you trade your lowest rated WR for him the trade will be accepted allowing you to go back in and edit Calvin Johnson back to his proper position of WR.