Release : 11.6
Doing an IPL is certainly not required when upgrading. If linklists are being used and the CBAALOAD library for release 11.6 has been recently added to the top of the list, then an IPL is needed to bring in the new library.
Otherwise, an IPL is not needed.
Certainly both release 6.4 and release 11.6 may be installed at the same time as testing continues with your 11.6 version and results are compared..
Please know that these should be installed into separate CSIs as there are liked named members in the 6.4 CAILIB and the 11.6 CBAALOAD.
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.