I am going to install Redhat AS 4.0 on AMD 64 bit processor while enabling the 32 bit option.
My question is, if I need to switch the processor to 64 bit in future, does the OS needs to be resintalled?
There is no switching/enabling of processor involved...AMD64 has the ability to use both 32-bit and 64-bit OSes. However, if you want use a 64-bit OS, you will have to install a 64-bit OS. Otherwise, AMD64 works just perfect with 32-bit support. I am not sure, redhat enterprise edition comes with 64-bit, however, fedora, ubuntu, suse do have 64-bit edition.