"Bill Denton" wrote

You can take a "no radio" aircraft and upgrade it to "full IFR
capable". I think this requires some sort of sign-off.

Radio installation has nothing to do with the aircraft being
certificated for IFR operation on its original type certificate.
"No radio" IFR operation in uncontrolled airspace is completly
legal.

I doubt that the type certificate enters into it at all...

The type certificate (or a supplement) is ALL that matters in
addition to the usual Part 91 transponder/static checks.

You will find that the POH or FM will list the approved types
of operation and for most production aircraft these include
DAY/NIGHT and VFR/IFR.

Bob Moore