I've read this is the case, but I wanted to ask people who actually use the gear for a 2nd opinion. I have an old Pur Hiker, and I need a new filter. Does the Katadyn Hiker pro use the exact same sized filter cartridge as that old Pur Hiker, or do I need to buy a whole new filter system?

An old trick for filtering particularly dirty or stagnant water is rubberbanding a coffee filter around the pre-filter. Adds a little life to your filter, if nothing else.