Lots of FAIL here.
Your Form 4 should show the barrel length, overall length, and caliber of the gun that was transfered to you.
Now that you own the gun you can change any or all of these descriptors legally. This is considered by ATF to be a temporary change no matter how long the gun is in the new configuration.
If you transfer the gun at some future point, Martinsburg will only approve the transfer if the gun is in the configuration listed in the registry, so you should keep all the original parts to return the gun to the original configuration.
Now, if you do something like sell the original upper, then you have made a permanent change. ATF asks that you advise Martinsburg if you make a permanent change. Before you can transfer the gun you will need to write a letter advising ATF of the new configuration so that it can be reflected in the registry. It takes about 2 years to get a response from ATF if you change configurations.
Don't ask me how I know all of this.
I will never make permanent changes again. I will always keep the parts to return the gun to the original configuration. It gets real confusing otherwise.
Now some guy is going to come along and tell you that what I said is BS and that you can simply list the new configuration on the future Form 4 and everything will be fine because it worked for him..
You can believe him or you can learn from my experience after my interesting conversation with a specialist and several more letters of explanation to ATF>
Just Sayin!