If you think that it takes a complete redesign of the core, then you would be mistaken.
It is also more complicated then you let on. It isn't a complete redesign, but there are significant hardware changes needed outside of the ALU and microcode. Read some of the AMD64 system manual. You'll realize many things need some hardware changes (new registers, tables, control lines, feedback lines) to work properly. The ALU and microcode changes are relatively easy. It is all the other fine details that are complicated.