The short answer is very simply - no!
There is no contractual commitment with a monthly plan. After your first months service you can choose a new plan with that carrier or switch to a new carrier plan.
All you need to do is arrange to port your number to the new carrier if you do choose to switch.
Remember that if you signed up to a finance agreement to pay for your phone by monthly installments you would need to pay this off before you can switch (as a general rule). This is where the carrier has a contract with you to repay the finance while you remain connected to their network.
Heres a selection of the best monthly wireless plans