Can you change any name at minecraft without paying?

For example, if I change minecraft names in the game, do I have to pay again?

You can change your name as many times as you like, but you have to wait 30 days each time until you can change the name again. But yes, it always works.

As Jakob has already mentioned, this is actually only once a month. But it does not cost you anything. Remember that you could get difficulties on servers. For example, in multiplayer mode, chests can no longer open from you, as they are secured to your old name. If you play freebuild and build somewhere. In such case, contact the server teams.