Justify introducing Lua at my workplace?

[Tim Hunter <TimHunter@...>]

I have an opportunity to introduce Lua in my workplace. My goal is to 
replace an existing scripting language. The existing language is 
proprietary, was developed in-house, has limited functionality, is hard 
to maintain and extend, and is only maintained by its inventor.

I have already shown that Lua can be used in place of the existing 
language, but my boss wants to know why we should bother. After all, he 
says, the existing language is working (for now) and we have several 
developers who are trained to use it. These developers would have to 
learn Lua.

Can you point me to existing web pages or other documentation that I can 
use to help craft my rationale?

Thanks!