We are running WAS 126.96.36.199 32bit I'd like to upgrade it to 64bit as one of our new apps requires 4GB JVM.
Do you think it's possible to achieve this kind of upgrade without reinstalling the product ?
I believe you have to reinstall. But, if you are running Network Deployment (DMGR), you can keep the DM and the Node agent 32 bit.
Assuming you are doing this on the same machine, just install 64 bit WAS in a different directory. During the install, select a custom profile so you just create a new Node / Server and federate it to the existing deployment manager. Once that's complete, you can map the existing applications in your old server to the new 64 bit server.