[PROJ] [Questions] Vertical transformations advices for PROJ >= 9.1 version

[Christian DOULIAC]

Hello, I would like some precision about CRS conversions.

I'm aware that there are some changes about vertical transformations with PROJ >= 9.1 as stated here : <https://github.com/OSGeo/PROJ/pull/3119> https://github.com/OSGeo/PROJ/pull/3119
Or here : https://proj.org/en/9.4/apps/cs2cs.html#cmdoption-cs2cs-3d "Starting with PROJ 9.1, both CRS need to be 3D for vertical transformation to possibly happen."

I tested it and I recreated the behavior of pre 9.1 and post 9.1 versions :

-PROJ 8.2.0 :
echo 0 0 1000 | cs2cs +proj=longlat +datum=WGS84 +geoidgrids=/proj-build/data/for_tests/egm96_15.tif +vunits=m +no_defs +type=crs +to +proj=longlat +datum=WGS84 +no_defs +type=crs -f "%.14f"
0.00000000000000 0.00000000000000 1017.16157913208008

-PROJ 9.2.0
echo 0 0 1000 | cs2cs +proj=longlat +datum=WGS84 +geoidgrids=/proj-build/data/for_tests/egm96_15.tif +vunits=m +no_defs +type=crs +to +proj=longlat +datum=WGS84 +no_defs +type=crs -f "%.14f"
0.00000000000000 0.00000000000000 1000.00000000000000


I would like to have some advice to get the past behavior, sometimes I need to do theses vertical transformations.
I have found some tips here : https://github.com/OSGeo/PROJ/pull/3119#issuecomment-1069272183 and here https://github.com/OSGeo/PROJ/pull/3119#issuecomment-1069272183
>From theses comments I understood that I need to promote theses CRS to 3D, more precisely the PJ_TYPE_GEOGRAPHIC_2D_CRS and PJ_TYPE_PROJECTED_CRS types.

Did I understood the comments right? Are there any other cases (types) that I should take care the same way? Should I only look for the type of CRS or is there other information that I need to look for?
My main goal is to get the past behavior.

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