I'm skeptical about the suggested change, for several reasons.
One reservation is that in my experience, 'one-trick pony' just isn't all that common, whereas 'nerd' is extremely common. 'One-trick pony' doesn't seem to be listed in NOAD, AHD2, Webster's unabridged, or Random House unabridged; and it wasn't in the OED under one-tr... as far as I could tell, though it could have been somewhere in the columns of microscopic print under 'one.' I've heard of it, but it probably wouldn't occur to me to actually use it.
In fact, I was thinking maybe it was mainly BE, which would fit with all CM2DD's examples in #9; so I was very surprised to see the 'chiefly US' marking in #10.
Another reservation is that I have the feeling the entry may be intended to reflect the sense used in combinations such as computer nerd, history nerd, math nerd. In that case the focus really is on a particular subject, though the word 'nerd' alone doesn't necessarily imply that in other contexts.
Above all, to me 'one-trick pony' doesn't necessarily have anything to do with knowledge, only skill, doing one thing well to the exclusion of all else. As far as I can tell, the one trick may not require any particular intelligence, it can just be a knack for something; whereas a nerd does by definition at least know a lot about a topic (in a boring way) and spend a lot of time studying it.
I also have the feeling that 'one-trick pony' may sometimes suggest a fairly brief appearance in the spotlight, more in the direction of a nine days' wonder, a flash in the pan, etc.