Como você diz, a FAA certifica uma aeronave para certos "tipos de operação". Para um avião de categoria normal certificado sob 14 CFR 23, ele basicamente diz que deve ser "estabelecido apropriado para o equipamento instalado":
§23.1525 Kinds of operation.
The kinds of operation authorized (e.g. VFR, IFR, day or night) and the meteorological conditions (e.g. icing) to which the operation of the airplane is limited or from which it is prohibited, must be established appropriate to the installed equipment.
O equipamento instalado necessário para o vôo IFR (além do necessário para o voo VFR) está listado em 91.205 (d) e é principalmente o equipamento usado para se comunicar com o ATC e manter o controle de o avião ao voar sem referência visual ao solo (portanto, nenhum equipamento de navegação é especificado):
§91.205 Powered civil aircraft with standard category U.S. airworthiness certificates: Instrument and equipment requirements.
...
(d) Instrument flight rules. For IFR flight, the following instruments and equipment are required:
(1) Instruments and equipment specified in paragraph (b) of this section, and, for night flight, instruments and equipment specified in paragraph (c) of this section.
(2) Two-way radio communication and navigation equipment suitable for the route to be flown.
(3) Gyroscopic rate-of-turn indicator, except on the following aircraft:
(i) Airplanes with a third attitude instrument system usable through flight attitudes of 360 degrees of pitch and roll and installed in accordance with the instrument requirements prescribed in §121.305(j) of this chapter; and
(ii) Rotorcraft with a third attitude instrument system usable through flight attitudes of ±80 degrees of pitch and ±120 degrees of roll and installed in accordance with §29.1303(g) of this chapter.
(4) Slip-skid indicator.
(5) Sensitive altimeter adjustable for barometric pressure.
(6) A clock displaying hours, minutes, and seconds with a sweep-second pointer or digital presentation.
(7) Generator or alternator of adequate capacity.
(8) Gyroscopic pitch and bank indicator (artificial horizon).
(9) Gyroscopic direction indicator (directional gyro or equivalent).
No que diz respeito à conversão de um avião de VFR para IFR, a Folha de Dados de Certificado de Tipo (TCDS) deve especificar que o avião foi projetado para voo IFR e, em seguida, você pode adicionar o equipamento ausente para torná-lo qualificado.