Are there any scenarios's where the license master is on lower end say 6.4.5 and all the splunk instance Search head , indexers , deployment server are on higher end say 6.5.1 and working with out any issues .
As per the document it says that " A license master must always be of equal or later version than its license slaves."
In your specific instance, a 6.4.x LM is only compatible with search peers on the following versions: 5.x, 6.0.x, 6.1.x, 6.2.x, 6.3.x, 6.4.x. As your "search peers" are not compatible, it would be advised to upgrade. If you "cannot", we would recommend that you move that role to another instance or stand up a new instance which can be at the same or higher level.