So, go to www.dropbox.com/account and downgrade, you need to wait for it to try and bill and fail and THEN say your account is downgraded (it may take a few days after billing). Then you can go to www.dropbox.com/plans and upgrade to correct place (and update address etc. in Account).
Your files ARE safe in this time its just syncing will stop - i.e. it wont add new or updates while you are over quota
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