And that's exactly what we do. Not our idea - the IBM as400 line (still sold and widely successful after several total changes to the underlying ISA) does the same.
Not only does the bit encoding change with new Mill models, but each family member across the line has a unique bit encoding, yet any load module runs on any present or future member without rewrite or recompile. Details (and a live demo where I'll ask the audience to create a new instruction, and 20 minutes later will write and execute code using that instruction on the Mill simulator) in the January talk - sign up for announcements at ootbcomp.com/mailing-list.
Not only does the bit encoding change with new Mill models, but each family member across the line has a unique bit encoding, yet any load module runs on any present or future member without rewrite or recompile. Details (and a live demo where I'll ask the audience to create a new instruction, and 20 minutes later will write and execute code using that instruction on the Mill simulator) in the January talk - sign up for announcements at ootbcomp.com/mailing-list.