[PROJ] Converting coordinates between different ellipsoids... possible? valid?

[Nyall Dawson]

Hey list,

If I attempt to create a transform using proj_create_crs_to_crs, with
the source proj string "+proj=longlat +datum=WGS84 +no_defs" and dest
proj string "+proj=longlat +a=2632345 +no_defs", I get the error
"internal_proj_create_operations: Source and target ellipsoid do not
belong to the same celestial body".

My question is -- should this be allowed? It worked ok in previous
proj versions, and it's a common situation in QGIS which I'm having
issues fixing after the v6 port.

Or is it violating some rule of transformation? How can I reproduce
the proj 4 behaviour here?

Cheers,
Nyall