NOTE: My recommendations below do not suffice as legal advice and is intended for informational purposes only. If you have specific questions about CASL please consult your legal counsel.
No, double opt-in is not required under CASL. However, we've made the recommendation that if a sales person or someone takes an opt-in over the phone, that this is likely the best way to handle it. Otherwise, it would come down to the recipient vs. the person on the phone, and the law will usually favor the recipient, unless you happened to have recorded the phone call.
There is no grandfathering under this law, UNLESS you ran an opt-in campaign and followed PIPEDA guidelines. As far as I'm aware, most companies didn't do that. So, yes, it is best advised to just run a campaign to get everyone to opt in again.
If you've recently gotten someone to opt-in and it's CASL compliant that you should be ok. As I mentioned before, from what we know they're really looking to go after true spammers or people who are not even attempting to abide by the law. So, more than likely you'll be ok
As mentioned above though, if you do have specific questions around your particular setup or practices, I would highly recommend running it by your legal counsel.