MySQL - Migrating from Master-Slave replication to Master-Master

We currently run 2 large mysql master-slave replication trees.

Each tree is built with one master, several 'middle' slaves replicating from the master, and 'end' slaves replicating from the 'middle' slaves. For example:

                              - end slave
            - middle slave  - - end slave

-  master - - middle slave  - - end slave

            - middle slave  - - end slave
                              - end slave

I would like, if possible to get rid of the master, and promote the 'middle' slaves to be the masters in master-master replication. Example:

                        - end slave
              master  - - end slave

              master  - - end slave

              master  - - end slave
                        - end slave

Is this possible? What would be the process I should follow? What sort of downtime should I expect?

Thank you very much in advance!