@Ralf Albers - Your paid features are attached to your team and not your Miro account profile. This means that you could all features available to you for boards that are in your team while at the same time you could also be a member of a Free Plan team with limited functionality. This is also true when editing boards that reside in a paid team that you are not a member of - in this situation you have the role of "guest editor" and have limited functionality, as outlined in the following help center article:

https://help.miro.com/hc/en-us/articles/360012524559-Collaboration-with-Anonymous-Guest-Editors#h_01ET753DDKH0KYG5ZEY8JBCTHV

In summary, you need to have a paid seat on each team that you need to have full editing feeatures in. However, you cannot pay for a your own seat on that team - the billing admin of that team will need to pay for you. And you definitely do not want to use the "upgrade" button for any Free Plan teams that you may be a member of. If you do, you would be upgrading all members on that team.