There is no automatic way to do that. You will have to create a new folder in maildir format and copy the messages from the mbox folder into this new folder. Remember to adapt any filter rules connected with the old folder before you delete it.
Non existe ningunha forma automática de o facer. Terá que crear un cartafol novo no formato maildir e copiar as mensaxes desde o cartafol mbox para este novo cartafol. Lémbrese de adaptar todas as regras de filtrado ligadas ao cartafol anterior antes de o eliminar.