As long as the ai aircraft and traffic.bgl are installed properly it should work. If you're downloading the files, unzip all files to temporary folders as locations in FSX are not the same as in FS2004. If you copying from your FS2004 install, just put them in the proper locations.

These are the  normal locations in FSX:

By default the ai aircraft go in ..\SimObjects\Airplanes\ just like flyable aircraft.

The default location for ai 'traffic*.bgl' file(s) in FSX is the ...FSX\Scenery\World\scenery\ folder. If you want to preserve the traffic file locations as they are in fs2004, so you can switch eras, you can. Just remember to add the respective Areas to your Scenery Library

Note that using fs2004 formatted traffic*.bgl files in FSX will disable the FSX default ai air traffic and the ships and ferries traffic. Only the traffic from the FS2004 traffic files will show. It will not effect the small boat, road or airport traffic.

If you want to keep the FSX default traffic, the FS2004 traffic.bgl files must be converted to FSX format. There are a couple of utilities that will do that. If you're interested I can post the links.

The CalClassic scenery and Afcad files are not compatible with FSX but the ai aircraft and traffic files should be.

Dave