Changing locks on a co-op unit.

You can have a locksmith change them to pretty much whatever you want, given any limitations imposed by the door configuration, or house rules about how door locks should look/work. Prices will vary significantly depending on what type of lock you put in (higher security keys = more money), and which locks: are you doing the deadbolt that is mortised into the door, or is this a surface mounted deadbolt, or both? Figure $300-500 all in.

If you're even slightly handy, you can save a lot of money by not having a locksmith come to you:
http://hubpages.com/hub/How-to-Change-Your-Locks

We recently learned that when you "change the locks" or "rekey the locks" that might mean that anyone with a master key can still use it to access your apartment. What our locksmith did instead was "change the cylinder."

Changing the cylinder is exactly what I did when I moved in. Five dollars plus five minutes with a screwdriver plus a couple of sidecars.

Use a solid screwdriver so you're not mixing vodka with brandy.

Thanks everyone for your help!

Any opinions on the best type of deadlock? We are looking at some of the more basic units (Kwikset) vs Medeco.