Is the upgrade in power allowed? I think it should be banned if there is no rule. And if it is banned should be stated in the rules

Upgrade allowed. I thought a lot about how to introduce upgrades into the rules, but I came to the conclusion that players would be uncomfortable using the game with such restrictions, especially since the levels open up gradually and become more difficult. Any category can be completed at any player level. If you improve a player, you can beat your own record.