Since you have a plumbing back ground this may be a very good field for you.
I was talking to a coworker who is one of 2 or 3 fire sprinkler system reviewers in the state. He happened to be talking about how shitty plans were drawn up for a job hes reviewing. The guy designing it has 0 experience with fire suppression but knows cad which is why they hired him. If you know cad, plumbing and could learn about fire suppression it is my understanding there is high demand for good people.