If you decide to cancel your plan within the trial period you will not be charged a thing. If you cancel your plan sometime in the future you will never be charged again.
No, there are no long term contracts or commitments. You pay on a month-by-month basis and if you cancel your account, you will never be billed again.
If you upgrade from a free to paying plan you will not be eligible for the free 30 day trial. The free 30 day trial only applies if you sign-up to a paid plan. You can always downgrade later.
Yes, simply click on the account tab whilst logged in and you will see your upgrade and downgrade options.