As far as I know, if you are included as a qualified LEO under that law, then you are what you are. There's no further qualification needed. You do of course have to carry the correct type of identification, which is included in the text of the law, and can be obtained from the agency that you are currently or were previously employed by. You may also have to qualify at a range to meet whatever specs other active officers also meet.
The best thing would simply be to talk to your supervisors, current or former, and there should be nothing to it.
I have used this guy for the past 2 years, they are few and far between. It would simplify a lot if an NRA Instructor were allowed to certify the qualification. As it stands now only CJSTC certified instructors can conduct the qualifications. This level is far above that of an NRA certified instructor.