Hey, thanks for joining our jam, welcome to the world of game jams.
It really depends on the jam, basically it is the honor system for most. for our jam it is built in to the rules...
Continuing an existing project is allowed, only if the majority of the submission is new content.
This allows a way for people to continue projects or reuse parts of previous without feeling like they are cheating.
I will say though, a big part of what is great about a jam is starting fresh on a new idea or major rework of an old idea. Also, two weeks is plenty of time to make something great and forces you to keep things focused.
My main advice with game jams is to experiment and try to make a fun cool new game.