Your understanding is incorrect. Nothing in law expressly creates such a rule.
Each firearm is classified by applying the definitions as set out in S 84 of the criminal code. Any modification to the firearm has the potential to move the firearm in to a more or less punitive classification.
There are a few characteristics that can determine classification status that even if changed will not result in a change of legal classification, for example, once a handgun, always a handgun (and therefore always at least restricted), and Once a Full auto, always a Prohib.
Firearms, other than handguns, that are classed as restricted strictly because of a short barrel length are usually open to moving to the NR class if you simply add a barrel of sufficient length.