unable to change to directory path unisims_ver.
Yes, I have ISE 4.2i, 6.3i, 7.1i, and 8.1i all installed. I simply change that XILINX environment variable when I want to switch between them.
Don't try to share anything between versions. Do full installations into separate folders.
I don't use Project Navigator. I build my projects using a command line makefile. That makes it really easy to switch a project between ISE versions.
If you use ModelSim, you need to compile separate simulation libraries for each ISE version. For convenience, I modified modelsim.ini and replaced lines such as this:
UNISIMS_VER = C:\Xilinx81\SimLibs\unisims_ver
with this:
UNISIMS_VER = $XILINX\SimLibs\unisims_ver