In the new condition, it is still worth CAD 20, as worn round - will be as 1/4 oz silver max.
I don't think that is true. As a 1/4 oz silver, it is worth $5 (assuming $20/oz CDN silver). Wear may take a few cents off that price as the weight drops below 1/4 oz.
The face value is $20 and will be for as long as the mint continues to accept them for redemption. They accept pre-1967 quarters for 25 cents as long as they are identifiable as a quarter...