Only by saving to 19 and then opening in 19 and saving to 18.
You can only save back one version at a time.
You might be able to download 19 from the 'MyArchicad' link at the top of this forum if you don't have it.
Barry.
One of the forum moderators.
Versions 6.5 to 27
Dell XPS- i7-6700 @ 3.4Ghz, 16GB ram, GeForce GTX 960 (2GB), Windows 10
Lenovo Thinkpad - i7-1270P 2.20 GHz, 32GB RAM, Nvidia T550, Windows 11