There are different solutions, I think. IMO the easiest way is to make sure all AutoCAD's run the same LISP-mode. You could have acad.lsp (uncompiled) to check lispsys and if necessary change it's value and restart AutoCAD.

Another approach could be to have two versions of every LISP routine - one for the 'old style' FAS files and one for the 'new style' FAS files. Give them a) different names or b) put them in different folders. And then again use acad.lsp to check lispsys and for option b) set the support path to one of the folders. For option a), use acaddoc.lsp to check lispsys and have the autoload function to load the appropriate file.

This could be a solution to know how was done, but I still need to know how to load it, been a normal FAS when lispsys is 0.
Otherwise I need to order all to set lispsys to