On ESPN, if you click on
My Team and then
Manage DL, you should be able to switch a new DL player for one already on the DL. You always have to
Save your DL moves. If you already have 2 on the DL, it doesn't really matter which one is on it, since you can have a maximum of 2 on DL, and any others are on your Bench or Active roster.
The player will remain on the DL until you move him back, even when his team has reinstated him.
Last year, ESPN restricted roster moves, if a player on DL had returned and you hadn't activated him. But I noticed that, in 2012, someone else was able to grab a Free Agent without moving an active player off your DL list. You couldn't do that last year.