What’s the difference between the “Community” and the “Support Contract” versions?

Presently, there are no differences whatsoever between the “Community” (that means Open Source and thus free to use) and “Support Contract” versions. They are the same package, available for download at https://zammad.org.

If you self-host Zammad without a support contract and later decide to purchase one, you may do so without modifying your deployment in any way.