Sounds a bit like a repogrammable ROM (which is no longer ROM in that case, ROM=Read Only Memory). Kinda what FPGA does, if I’m not mistaken (and what you were referring to, right?).
If you take a look at the die you’ll find what is in effect a ROM on board, a look-up table defining what each instruction does. A machine with said capability can change this ROM, and not merely emulate a different instruction set, but be that instruction set.