Slight correction on your facts...
The "once a X, always an X" applies only to machineguns (and some AOW and some post ban evil assault rifles).
You *can* write ATF and let them know that you have changed the configuration of a short bbl rifle or short bbl shotgun and that you would like it to be removed from the registry. Obviously this would be for a perm. change.
If you want to slap a 24" bbl on your SBS and go duck hunting with it, there is absolutely no problem with that.
Interesting comment "you must still keep a copy of your Form 4 with you". Interesting I say because there is NO federal requirement that you must have your paperwork with you when you're out shooting your guns (machinegun, whatever). Some state laws require you to have the paperwork, but a lot do not. And, again, there is no federal requirement to have the forms with you. Prudence dictates, however, that you keep a *copy* with you while shooting in case you run into problems with the local law.
danbrew :->