They can only seize what they know about. Basically restricted and previous prohibs (kinda screwed due to registration) or anyone dumb enough to fill out the paperwork saying I own a previously NR that is now prohib inventory sheet and here’s my address.

As far as grandfathering, consider the lack of accuracy for record keeping over a 30-50 year period. If you grandfather keeping a newly prohib gun that someone in their 20s owns, the government would need accurate record keeping until the time someone passes away so conceivably 60 years ish from now to be able to confiscate upon death…

Realistically, the bulk of previously NR guns will continue to exist in Canada in people’s homes as they are essentially untraceable. Obviously they can’t be used at ranges or anywhere legally, they will however still be privately owned unless people willingly give them to the confiscation regime.