Why do they need to change it? If you owned the company would you want stoned employees?
I don't think it's any worse than taking anti-anxiety medication or pain killers at work, which as long as you have a prescription it doesn't count against you for employment. THC stays in the body for a long time; i just think it's silly that people can drink themselves to sleep and take pain killers, opiates, other anxiety medications that really mess up your mind, but THC is still banned. If it's legal to use it, there is no test that can tell you if they took it before bed or on the way to work, just like pills.
I work with a young guy who's jacked up on Adderall and Xanax all day long, he's very open about it. He has prescriptions for it and all HR can do is tell him not to talk about it, but in reality he can if he wants to. He's actually a pretty good worker and gets all his work done, so no reason to complain really.
A stoned employee wouldn't bother me too much, i work in Engineering which i've had a lot of CAD drafters and designers that i knew smoked weed. Never had a problem with it, just make sure you check all their calculations, which i have to do anyway.
