I'd say about a year is all you can get out of them. After that they tend to make the algae grow more than the corals. (I've been told that it is due to the color spectrum changing over time) So when the algae grows you need new bulbs.
Make sure you take in the bulbs when you go to get new ones. That way you know exactly what kind to get. I also recommend saving the old bulbs, as backups, in case one of the newer bulbs burns out. You can even put a date on the old bulbs to remind you when you replaced them.
As for upgrading the lights... I think the only real way to upgrade is to remove the cover and hang or clip on new light, but I'm not sure. There are some mods out there that can replace the PC with MH, but again I don't have the specifics. I had looked into it a while ago, but it was just as much to mod the hood as to buy a whole new setup.