The level of concern is a question of public policy, which, again, is in your domain.
However, I was involved at the time in the creation of the magazine regulations. The government policy of the day, as explained to me, was that the concern was to provide a quick and easy way for owners to come into compliance with the magazine regulations at the time the law changed. As a result, the conversion methodology of magazines was not all that strict.
A magazine can be modified from a prohibited capacity to a non-prohibited capacity through the use of a pin or similar device, which does in fact block the magazine, but it's not incredibly secure.