Being Non-restricted, there's no reasonable way for the RCMP CanadianFirearmsProgram to notify the owners. If they do it'll be a political mess for the RCMP, because they'll be admitting they kept a copy of the LGR, in violation of the law.
Assuming the 4% of PALers who signed the petition represent the readership and reach of various social media, my guess is that 96% of CZ858 owners will not find out about this date until it's too late.
The bulk of them could be notified via the vendors that sold them, but that's a lot of work for the vendors. (I'm assuming they have emails, otherwise its envelopes & stamps)
Low Estimate
CZ858 9800 * $729.95 = $7,153,510
Swiss 1800 * $2999 = $5,398,200
Total: $12,551,710
High Estimate (most CZ owners I've seen, an old NFA announcement; and higher Swiss Arms price)
CZ858 13000 * $729.95 = $9,489,350
Swiss 1800 * $4000 = $7,200,000
Total: $16,689,350