I had previously been under the impression that if a pilot held an instrument rating that it would apply to aircraft in another class. The requirements of 61.57 would seem to support that since currency can be maintained in aircraft of the same category.
However, reading section 61.133 leads me to believe that any check ride to add a class would require an instrument checkride as well:
A person who applies for a commercial pilot certificate with an airplane category or powered-lift category rating and does not hold an instrument rating in the same category and class will be issued a commercial pilot certificate that contains the limitation, “The carriage of passengers for hire in (airplanes) (powered-lifts) on cross-country flights in excess of 50 nautical miles or at night is prohibited.” The limitation may be removed when the person satisfactorily accomplishes the requirements listed in § 61.65 of this part for an instrument rating in the same category and class of aircraft listed on the person's commercial pilot certificate.
Am I correct in this interpretation? Are there any other CFR sections that might shed some clarity on this? Does any class rating add-on, (presuming one wanted to exercise the option of IFR in the new class) require a full blown instrument check even if one is already instrument rated and current in a different class?
ADDENDUM: After re-reading this almost seems self answering based on the quote I provided, but I know what sometimes regs can be interpreted differently based on other sections. I am just incredulous that a full-blown syllabus per 61.65 would be necessary when really IFR isn't any different from Single to Multi. Makes the upgrade that much more pricey!