The Registrar of the Canadian Firearms Program is loathe to change the status of Prohibited handguns without having a proper >105mm barrel attached. Even if a gunsmith removes and destroys the original short barrel, the farthest they will go is frame-only — Prohib. Nothing in Law makes this mandatory, but it's how they operate.
If it's available as an option to your 'smith, you could theoretically attach a long barrel from a donor gun, submit it to be Registered as Restricted, then remove the barrel and give it back to the donor. These sorts of shell games would not be worth your while on this lot of low-value guns, I would think.