Why Convert All Spots to CMYK, rather than RGB?
Please read mrserge's answer and the question there before reading this question. mrserge answered
What we will do: convert everything to CMYK, then replace specific CMYK value with other and we will do that by using standard Acrobat Pro Fixups.
3. Search for Convert All Spots to CMYK and run Fix. Now all colors in your document are CMYK.
What changes, if anything, if you Convert All Spots to RGB?