That is because the PLN file type is associated with he last installed Archicad version.
Try right clicking on the file name and choose "Open With...".
If the version of Archicad you want is not in the list, then you can browse to find it (and it will be there next time).
You can choose to re-associate an older Archicad program to always open those PLN files.
But as far as I know it is not possible to have version 20 open in 20 while 21 opens in 21 and 22 opens in 22 etc., just by double clicking the file.
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