brotherhood_of_LAN - 10:20 pm on Aug 3, 2011 (gmt 0)
Hard to say. Removing it puts you at risk of having duplicate values for the current PK as auto_increments have to be an index of some sort (in this case, a standard INDEX that accepts duplicates).
If you don't foresee that as a problem and you don't regularly query the current PK row then feel free to give it a try. If you don't reference the current PK field in INSERT's to the table then the duplicate problem should never arise.
If the table's primary role is to store unique $term/$url combinations and the current PK is merely a more compact reference to it, I'd say change to the new PK.