In what way "worn out"? Buttons unresponsive? Are you sure it isn't just the batteries (the lights are supposed to chase when the batteries are getting low, but I have found they don't always - the relevant light just shows a bit dim)? There are instructions on the wiki for disassembly and contact cleaning: http://wiki.hummy.tv/wiki/Remote_Disassembled
I've had these handsets for five years, and none of mine have worn out. The worst that has happened is that the chrome plating on the nav pad has flaked away, leaving a sharp edge (apply clear nail varnish to protect it), and the OK button is a bit unresponsive (should be OK when cleaned).
The subject of universal remotes already has a topic in the 'Arms: http://hummy.tv/forum/threads/universal-remotes.1479/
The fact is that, unless you get an all-singing-all-dancing universal remote (totally configurable and at considerable expense), there will always be some buttons missing. I also use a Harmony 300 in a couple of locations, it's pretty good and can control four items including a TV, but with only four "undedicated" buttons I have to sacrifice some of the standard RM-F04 functions: like bookmarks, for example.
Check out the first link in post #2 that describes taking the remote control apart. Where you have poorly responding buttons, the method described that uses foil and double-sided tape does work.The PVR and TV mode buttons have to be pressed down really hard with finger nails to get a response. Other buttons mostly OK. Its over 5 years old and batteries are fine. I read somewhere that these remotes are prone to this and 5 years seems to be a better than average life for these remotes.
I'm surprised that your PVR and TV mode buttons are giving you trouble, as these are the least used buttons on my remote controls. It seems to be more common for the OK, OPT+, and Skip Buttons to give trouble, as these seem to get the most use.The PVR and TV mode buttons have to be pressed down really hard with finger nails to get a response. Other buttons mostly OK. Its over 5 years old and batteries are fine. I read somewhere that these remotes are prone to this and 5 years seems to be a better than average life for these remotes.