I believe changing jobs is easy BEFORE the first contract becomes valid. In other words, you can change your commitment before the starting date on your contract. That's why the recruiters wouldn't leave you alone before your actual start date but now have gone cold as ice. If you want to change employers now, you have to go through the government AND cancel your contract with your current employer. He's probably gonna want some reimbursements (visa/air fare), so it won't be cheap to do it.

Always be SURE of everything before signing your John Hancock on any contract.

Good Luck