The act is silent on that matter. The act talks about the registration of a lobbyist, not about the lobbyist's previous career. When they stop being a lobbyist, there is nothing to stop them from carrying on a different type of career.
There are other conflict of interest codes that the Ethics Commissioner looks after that may talk about some circumstances, but the Lobbyists Registration Act, which is my area of responsibility, doesn't speak to the point that you just raised.