While these people may be arrested and their cameras may be seized, they could easily sue for unlawful arrest if they were more than 8 feet away at the time. Exactly the same as if their cameras were seized before the 8-foot law were in effect.
If police unlawfully seize and destroy your recorded evidence, then it doesn't matter whether there was a law like this in effect. The officer could make up any story to justify their action under any law.
Mandatory body cam laws would be a lot more relevant in that case, not 8-foot citizen recording laws.
