Version 19.2 and later files can be opened by AGi32 version 18.x, however, please be aware of the following caveat.
If texture maps have been modified in terms of reflectance and/or color in AGi32 version 19, they will NOT retain these modifications when opening in version 18. The texture maps will revert to the original calculated reflectance/color. This will necessitate a re-calculation to correctly consider the new texture reflectance/color. IF YOU DO NOT RECALCULATE THE VALUES WILL NOT BE CORRECT!