When in doubt, visit the developer's website, where an FAQ says:
C"an we have a free Unity Pro license, make deferred payments, or have a special discount?
"Sorry, we do not give away Unity Pro licenses for free nor offer a delayed payment plan.
But we do care about developers of all shape and size, that's why we made a free, basic version of Unity available to the public. Projects you make with the free version are fully compatible with Unity Pro if you ever wish to upgrade later"