As soon as you put the 18.5" barrel on its NonRestricted and there is no longer any requirement to do anything in relation to a certificate.
The only reason to notify the CFP is to help them clean up their database and have them cancel the certificate. The certificate itself indicates a requirement to report, but that requirement is in relation to a firearm that no longer exists and is meaningless.
So ask yourself if it is worth the bother to call the CFP and suffer some ass pain trying to help them unregister a gun that doesnt exist? Some people will say yes for their own piece of mind. Not this guy.
There is no offence for having a certificate issued against your license for firearms that don't exist, and you cant be charged for NOT possessing a non existent firearm.
Wrong.
You have a restricted rifle registered in your name. You can't just put an 18.5 inch barrel on and call it a day and pretend it never existed.
You also can not just put on an 18.5 inch barrel and then go shoot it away from a certified range. Until it has been reclassified and removed from the registry it is still a restricted firearm. The 30 day grace period is in place to do take it to a certified verifier or gunsmith to get the inspection and paperwork done and not as a open window to do what you want then change it back before the 30 days is up.
You must have the rifle removed from the database or as Brian states, when they come to take it away and you can't produce it you're going to have a long day in a cell answering questions about where it is and who you sold it to.
If you sell it you will also be in violation of selling a restricted firearm illegally if you sell it with no transfer to someone claiming it's non restricted. You are putting yourself and the buyer in a bad spot legally. What is the new owner supposed to say to the RCMP if he gets stopped and they run the serial number and it comes up as restricted? Now he's in crap for using his rifle away from a range and you're probably going to lose your PAL for selling firearms illegally.
That rifle does still exist and just because you physically changed it to fit the non restricted criteria does not make it non restricted.
Be careful who you listen to on this site, I've read people post similar comments to yours and they were wrong as well. The rifle is a registered restricted firearm until the paperwork has been finished reclassifying it and removing it from the registry.
I emailed back and forth with the firearms lab about this when I first got my ACR as I wanted to have a non restricted rifle that I could also throw a short barrel into when I was at the range and the answers I got led me to sell my 12 inch barrel as it would have been such a pain in the ass to do it legally it wasn't worth the trouble.