I just tried this too and it seemed to work fine.
One minor thing was that the 3D window was set for OpenGL in 13, but the project saves-as-12 opened with the Internal Engine for the 3D window.
One interesting thing is that when saving as 12, an LCF was created next to the 12 pln file which contained all objects that were embedded in the 13 file, and that was automatically loaded when I opened the project in 12.
Karl
One of the forum moderators
AC 28 USA and earlier • macOS Sequoia 15.2, MacBook Pro M2 Max 12CPU/30GPU cores, 32GB