1. on all replicating slaves issue:
STOP SLAVE;
2. on the slave that will be master there should be no binary logs floating around. issue:
RESET MASTER;
3. change the ip address of the new master to that of the old (or not, and update your clients instead)
4. on the replicating slaves issue
CHANGE MASTER TO (with connection info but no log file or position)
then
START SLAVE;