Yes. As soon as you sign-up you can easily upgrade to Premium or Enterprise Tiers using a credit card.
You can cancel anytime from within the management console. As soon as you cancel, your license is reverted back to a free tier. None of you projects gets deleted, and remain accessible to you, even if they are created outside your quota.
No-one’s asked. Hey, if you are unhappy with our tools, we’ll refund you.