That's not really true- they can't flip a switch and have everyones bitcoin client go *poof*. They push out a new update to the client, which in the testnet part of the code gives it a new genesis block which starts a new chain. The old chain still exists however. It works like bitcoin, they can't reset your bitcoin either. I would prefer they DONT reset the chain so that people can create longer term experiments, however if they did, worse case scenario, you could still stay on testnet version 3.