Well, I just found a way to contact Miro Support and had addressed my question to them.

But if you have the same experience - upgrade and downgrade to free plan and then boards are missing. please let me know if your boards came back or not. I need some good news to get rid of my panic mode :( 

@hsin li - What can typically happen in the downgrade-Team-to-Free-Plan process is that any boards that were “private”, i.e., the Team-level access for a board is set to No access:

Once a team is downgraded to a Free Plan, those boards become “locked” and are likely only visible tot he board owner, who will need to open those boards and set the “Team” access to “Can edit”.

So, you will either need to reach out to any team members and ask then to unlocked the board by setting the Team access to “Can edit” or remove those team members from the team at which point you will become the new board owner and could set the access yourself. You could then add those team members back to the team.

More on locked boards here → https://help.miro.com/hc/en-us/articles/360017731453-The-Board-is-Locked

Thanks for the explanation. The issue has been solved!