ADE should be quite safe to edit AFCADs, it should be enough edit the parking size and save it back.
Just check that, when ADE compiles the file as .BGL, you save it as AP_KDFW.BGL, not the name that ADE suggest and save it in the fsdreamteam\KDFW\Scenery folder. This will ensure you'll not end up having a duplicate AFCAD for KDFW, which creates all sort of issues.
And, it's best if you do a backup in a different folder of the original AP_KDFW.BGL that comes with Dallas before doing any modifications.