Just to elaborate you are changing the URL structure and if the extension author decides to not support it in the future you may be left supporting it yourself when you go to update phpBB. The other issue is if phpBB implements "friendly URL's" in the future the structure of the extension may not be the same as the what is implemented by phpBB.
“Results! Why, man, I have gotten a lot of results! I have found several thousand things that won’t work.”
Attributed - Thomas Edison