In fact, what happens most of the time when you take a micro-organism out of its natural environment, like a human or an animal, where it is infectious, and you passage it in vitro, it loses its ability to cause infection.
In fact, what you would see more often is the other way. You have a virulent micro-organism that becomes avirulent by being passaged in a lab, and that's what happens most of the time. Most of us work with strains that have lost their virulence because of being passaged in vitro. Many vaccines are based on micro-organisms that were passaged many times in vitro so they have lost that virulence, and they can be used as a vaccine. You inject them, and they don't cause any harm except that you get protection from a subsequent challenge by an infectious form of the pathogen. Unless you create a Frankenstein, a monster, there's no way you can have a level 2 pathogen becoming a level 3 pathogen. The other way around is much more likely.