You will be a felon when the grace period ends if you are ever caught with the magazine. You'll have to at minimum sell off the magazine or destroy it.

Yes, the law will affect nearly all RI gun owners. r/RIGuns has fairly regular posts updating some of the current legal challenges to the new law but those have not resulted in any stay or anything yet so as of today Dec 21 is still the last day to own it legally.

The law mentions that magazines can be "irreversibly modified" to hold 10rds or less, but there is no clear indication as what would be allowed as "irreversible" so basically you could try to modify it and then see what the court says if anyone feels like pressing charges against you to see what will stick. If you're not white I wouldn't suggest you try that one out with the police / courts.