If you are using a Free Plan, you are better off to have someone on your team create a new Miro Free Plan team using any email address they have never used with Miro, then have them invite you all to that new team (including their current email address) - now you are all members on both teams (except for the old admin).
Next, you open all boards from your old team and duplicate them I to the new team.
Finally, you can all leave the old team. Now you have anew team with all of your boards in that new team - you ahod also have the Team Admin of your new team promote someone else to the role of Team Admin.