If it was me I wouldn't even try.
The only way is from version 19 you can 'Save as' a version 18 PLN.
Then open in 18 and 'Save as' a version 17 PLN.
Then open in 17 and 'Save as 16 PLN.
Then ....
I think you get the idea .... until you get to version 11.
I doubt you would have all the versions installed to allow you o do this.
And besides there will be a lot of information from the new features in 19 that would be lost in the old version 11.
Best for your colleague to upgrade.
The only other option may be to export the 3D model to IFC and then import that in 11 - that is if 11 can even do that - I really can't remember and I doubt even if that would work.
Barry.
One of the forum moderators.
Versions 6.5 to 27
i7-10700 @ 2.9Ghz, 32GB ram, GeForce RTX 2060 (6GB), Windows 10
Lenovo Thinkpad - i7-1270P 2.20 GHz, 32GB RAM, Nvidia T550, Windows 11