Do what I have done. Sell it as a receiver only, no barrel length on the new registration. The barrels are non registered so the buyer has bought a receiver and separate barrel. Problem solved.
Kim
There are reporting requirements for some changes.
One requirement is to report when a receiver becomes capable of discharging ammunition.
So your method does not solve a problem, it potentially creates another.
It is a requirement to report certain changes within 30 days. The requirement is not to report changes which still exist after 30 days. Make one of the listed changes, and it must be reported. The 30 day limit is for reporting the modification, not for the duration of the modification.
As far as barrel cutting goes, if a barrel is cut to below the specified minimum, the firearm becomes prohibited. It does not matter if the barrel was below the minimum to begin with. 20" barrel cut to 16" = prohibited firearm. 16" barrel cut to 15" = prohibited firearm.
A cut rifle barrel is not a prohibited device. A rifle with a cut barrel is a prohibited firearm.
Was a short barrel manufactured at that length, or is it a cut down factroy barrel? Anyone who thinks it would be impossible to determine the difference is kidding himself.
Something else to consider - a longarm with a much abbreviated barrel does stand out. It will attract attention and questions. Might be a RO as in the OP's initial post. Most clubs do not want illegal firearms being used; this is likely what was in the RO's mind. In the OP's case, the RO was in error.
Had the OP encountered a LEO, instead of a RO, clarification of the legality of the fiream could have been more serious.