When flying VFR on top, what altitude changes must be reported?

When flying VFR on top, pilots are required to report any altitude changes during operations that fall under instrument flight rules (IFR). This is critical because, despite operating visually, the aircraft is still subject to IFR regulations when on top of clouds or in similar conditions.

Reporting altitude changes while in an IFR environment ensures that air traffic control (ATC) maintains awareness of the aircraft's current altitude. This is essential for safe separation from other aircraft and to effectively manage traffic in the airspace. Furthermore, any altitude deviations or changes can help prevent potential conflicts with IFR traffic operating below or near the flight path.

The other responses you encountered do not address the requirements stipulated by ATC for altitude reporting while under IFR, especially considering the circumstance of VFR on top where VFR traffic may not see the aircraft and reliance on ATC communication is paramount.