Edit:
Well, I didn't realize this was several months old when I posted! My bad...
Nope, cartridges are chipped and will not reset back to 100%.
If there's a fair amount of ink in the cartridge, you can continue printing, sometimes for weeks. I won't suggest this to anyone else, but I do it because I know this machine very well (been doing this since new in '05). You can clip the plastic tab on the ink cartridge and run it for another few weeks.
I also use a syringe and pull all the ink out of the old cartridge and put it in the other used cartridge in the machine (not the new one I'm installing). I've pulled 70cc's of ink out of "empty" cartridges, and almost always at least 30-40cc's.
Of course, I watch my prints very carefully and don't do long print runs if I know a cart is really low. I know how much printing I can get away with by flipping the cart up and down and watching the tab's movement.
If you don't want to be a tight-ass like I am, just replace them and not worry about damaging a print head by doing what I do. I haven't damaged one yet, but some day I may push it too far...