How to change ThinkPad's key binding

My ThinkPad has two keys originally intended as Back/Forward actions e.g. in web browser.

As I find them not very usable for browsing, I was searching for a way how to give them much useful role: eg. Page Down/Page Up.

My Ubuntu uses XKB, so here is the receipe:

1. open /usr/share/X11/xkb/symbols/inet
2. change lines
   key <I166> { [ XF86Back ] };
   key <I167> { [ XF86Forward ] };to
   key <I166> { [ Prior ] };
   key <I167> { [ Next ] };
3. restart the computer