@Boris Magas - While signed in as your paid Miro account profile, open the board(s) that are in your Free Plan account team, click on the board name/title (at the top-left corner of the board), and the use the Duplicate action.
@Boris Magas - Another way to copy a board from a Free Plan team to a paid team is to
1. add your paid account profile as a team member on your Free Plan team (which it sounds like you have done as you are the board owner)
2. right-click on a board that you want to copy over and use the Duplicate action
3. the board may open (if so, close it)
3. right-click on the duplicated board and use the Move to account action and select whichever team you want to move the board to.