Awijit,
I think, I found the reason for this behavior. I installed the AA in E:\ drive instead of traditional C:\ drive. But 9.0 adapter installer has placed .com.zerog.registry.xml file under "C:\Program Files\Zero G Registry" folder. After I removed the 9.0 file and run 9.0.1 installer, I am able to upgrade to 9.0.1 version.
I also opened the support case for this. Anyone who installed Adapter in other drives than C:\ drive might face this issue.