"Must be launched again in two weeks" what % can b

One could just keep a spare part for every piece of the vehicle and just replace each piece (or even just the major ones). Is there a rule against this? What is the limit to replaced parts?

Well, the rules call for replacing no more than 10% of the dry mass (i.e. not including fuel) between flights.

But it will still be a good idea to have a spare part for each system. Then, as long as the affected system weighs 10% or less, just replace it.

Sounds like Jason and his ship.

A lot of teams plan to replace ballons, parachutes, nose cones, steering vanes etc...

The aim of the X prize is to end up with a rocket you can land, gas up and fly again. The two weeks is required in new craft to pull the whole thing to bits and check nothing broke and then put it back together. Then pull it apart again and figure out where all the leftover bits came from...