If you want to make a permanent change to the SBR or SBS, it should be removed from the NFA and will not then be regulated under the NFA. If the owner maintains possession or control over the short barrel riffle or shotgun they should be careful about constructive possession of an SBR or SBS. While there is no requirement to notify the ATF of the transfer of an item which has been removed from the NFA, the ATF does recommend that you notify the NFA branch of such changes in writing so that the possessor is not mistakenly identified as the owner if the firearm is later used in a crime.
Until the firearms is removed from the NFA registry, it will have the same restrictions regardless of the current barrel length. You can not put a longer barrel on to bring to a state where a short barrel is permitted or avoid the necessity of a 5320.20 to cross state lines. Once a SBR or SBS it will always be a SBR or SBS until the item configuration is changed and the gun is removed from the registry. If removed, you can not the original Form 1 to change it back to an SBR or SBS at a later date