Release : 11.6
Please keep in mind that release 11.6 in compatibility mode shown as 11.6-C should give the same results as if running release 6.4.
So, there really is no need to keep 6.4 around.
But, it is certainly fine to do so while testing and getting used to the new release, even in compatibility mode, and to compare results.
Just please make sure that 11.6/11.6-C is installed into a separate CSI.