Three plans
A professional account comes with a free plan, and two paid plans are available on top of it. What each plan contains, and what it costs, is shown on the plans page in your area: these may change, and that page is the only up-to-date source.
What changes between plans
Plans differ by the features they unlock and by how much storage you have for your documents and your project photographs.
An account without a paid subscription stays fully usable: it is never blocked, only limited.
Changing plan
You change plan from the plans page. Moving to a paid plan starts the matching payment process.
When the subscription becomes active
A paid plan is only active once the payment has actually been collected. Depending on the method you choose, activation is immediate or follows validation — see the article on payment methods.