Scripts for beginners.

Why use == instead of =?

Use = for assigning a value, and == for comparison.

And what needs to be done to make the script run constantly (while the building is constructed), and not just once?

Every day, perhaps? Then you can use Trigger(NEW_DAY_TRIGGER,...). Otherwise, I don't understand.
If you need to constantly replace archers with crossbowmen, just remove the break from line 32 of the script.