The first half of my sentence, remove them as encountered from all those that did not register. Enforcement without registration is not feasible. Consider how enforceable the mag ban passed here would be if there was a list of allowed owners. Who cares if the mags are the ones they had when they registered, by registering they effectively signed up for a permit to possess "high cap" mags.
The roundup is the next cut, first get the "illegal" ones.