@Magdalena - If you are not using the same email address for both accounts, then you could sign into your Free Plan account, click on the Teams settings/gear icon at the top-left of your dashboard, go into Active users, add your other account (by email) to your Free Plan.

Once you have done this, when you sign into your paid account, you will see your Free Plan team listed on the left-hand side of your dashboard:

Now you can use the board menu ellipsis and selecting Move to account:

Note: If you want to keep a copy in your Free Plan, Duplicate it first:

Thank you so much, have a great day :)