Note: Strictly speaking, my answer below doesn't answer the question asked, because what the OP was referring to was the instructions reserved for customer use, rather than those it reserved for DEC, which is what my reply was about. The corrected version of my answer, for the "XFC" opcodes (with a prefix of hex 0xFC) would be not as far as I know.
I think you may be referring to opcodes originally labeled "Reserved to DEC". There were 3 of them, ESCD, ESCE, and ESCF, where in hex were FD, FE, and FF.
The later VAX Vector Architecture used ESCD as a prefix for two-byte opcode, but was only available on certain models, but I think this meets what you are looking for, but it wasn't just a microcode upgrade.
Because of the byte-order convention on a VAX, these two-byte opcodes were generally listed as words with the FD at the end, rather than at the beginning. For example: A1FD became VSMULL.