When I bought my conversion kit (.40cal.) for my Jericho 941FS 9mm. I was required by Magnificent to have it licensed.
Like Allegra said ,the barrel is considered a firearm. It is one of the questions given when applying for a Gun Safety Certificate.It's also the reason why we cannot order barrels online.
When I was active in the shooting sport, I usually indicate the true caliber of my firearm as shown on the PTT. Even if it's only a liability waiver, you'll be in trouble once something unforseen happens. I usually get a separate PTT for my conversion kit (already installed on the frame).If you intend to shoot a .40cal. out of your .45cal gun you must have the proper documents otherwise it's a Possession of an Unlicensed Firearm.Don't rely on the authorities not being able to differentiate between the two, if you encounter a knowlegeable cop (which for sure there are) you'll be in real trouble.Stay safe.