Slight problem with "unlearning" skills. The skill square that the unlearned skill once occupied apparently cannot be used again. Ever. So there is definitely a limit on how many skills one can unlearn/learn.
It just leaves the old icon there.
Click the old icon (The picture a skill creates in a square), and then learn a skill like normal.
If you want to revert to a blank square (like nothing was ever there) click the icon, then go to the far right of the window and there's a "Change Icon" box.
You can make all your skills look pretty cool with that (i.e. know what they are without hovering).
Edit: (One of the 'new' icons is the original blank space)
Hope that made sense.