OK, so I finally got an AR-15 
Have been wanting one for years, but restricted, blah, blah, blah.
The registration slip gives the barrel length. What if I want to put another upper reciever on it? I want to also get the M4 length upper, as well as others. (at some point will get another lower as well....)
Am I suppose to call the CFC, tell them I put on the 14.5 upper, shoot for a half hour, put the 10.5 one and call them again??
Then an hour later, put on the 20 inch upper and call them yet again??
I know this is not the case. If it is, very stupid indeed, like most of our gun laws. I don't want to fall on the wrong side of the law by accident. (or on purpose for that matter).
I vaguely remember something about 10 or 30 days of a change to notify the CFC. So long as I put the upper on for which it is registered back on within 30 days I am good?? When I store it, I should put on the proper upper?
The beauty of the AR platform is that it is so modular. A gun-controller's nightmare I am sure! Hence partly my reason for wanting one...
I am sure this had been answered here somewhere, but I can't find it.

Have been wanting one for years, but restricted, blah, blah, blah.
The registration slip gives the barrel length. What if I want to put another upper reciever on it? I want to also get the M4 length upper, as well as others. (at some point will get another lower as well....)
Am I suppose to call the CFC, tell them I put on the 14.5 upper, shoot for a half hour, put the 10.5 one and call them again??
Then an hour later, put on the 20 inch upper and call them yet again??
I know this is not the case. If it is, very stupid indeed, like most of our gun laws. I don't want to fall on the wrong side of the law by accident. (or on purpose for that matter).
I vaguely remember something about 10 or 30 days of a change to notify the CFC. So long as I put the upper on for which it is registered back on within 30 days I am good?? When I store it, I should put on the proper upper?
The beauty of the AR platform is that it is so modular. A gun-controller's nightmare I am sure! Hence partly my reason for wanting one...
I am sure this had been answered here somewhere, but I can't find it.