Since a binlog will have a specific format at the moment you do this, you may decide not to gamble with the two formats together although MySQL (eh Oracle [still can't roll off my tongue]) built this feature.
To play it totally safe without a mysql restart, try the following:
FLUSH TABLES WITH READ LOCK;
FLUSH LOGS;
SET GLOBAL binlog_format = 'MIXED';
FLUSH LOGS;
UNLOCK TABLES;
This will leave the last binlog in the 'MIXED' format. The penultimiate (next to last) binlog exists merely bring closure the last binlog that was in the previous format.
All existing sessions prior to the first FLUSH LOGS; will start writing in the last binlog once UNLOCK TABLES; is executed.
Give it a Try !!!