Something different ... huh? As I understand (and I am definitely no expert), the current 64 bit code is an extension to the 32 bit x86. That is why we sometimes see the comparison of x86 and x86-64 instead of just simply x32 or 32 bit and x64 or 64 bit. The processor i my first computer was a 286 (or 80286) and was considered a 16 bit processor, whereas the 8085, 8080, and 8008 were 8 bit processors. However, the 80286 was, according to this web site:
"designed so that assembly language for the 8085, 8080, or 8008 could be automatically converted into equivalent (sub-optimal) 8086 source code, with little or no hand-editing. This was possible because the programming model and instruction set was (loosely) based on the 8080. However, the 8086 design was expanded to support full 16-bit processing" see:
Intel 8086 - Wikipedia, the free encyclopedia
Even the 64 bit processing is "a 64-bit extension to the x86 instruction set" see:
x86-64 - Wikipedia, the free encyclopedia
One way of looking at it: in order to win the race to sell computers, the cheapest, fastest way was to remodel. Like starting with a mobile home, adding a "double wide" extension, then a front porch, and last a back porch. In the end though, you still have a patched together mobile home.
There are, and were even at the time, true 16 and 32 bit processors, but their code and structure was not compatible with the 8086 computers of the time, so in the end, Intel won the race. Even today, there is a true 64 bit processor, the Intel Itanium (formerly IA-64). But again, the "native instruction set level " is not compatible with the computers you and I are using.
What would be truly different would be to have an OS based not on 8 bit expanded to 16, expanded to 32, expanded to 64, but real 64 bit from the ground up. All new.