I'm pretty sure you can upgrade at the trainer eventually. Other then that, you can't get rid of your mission skill for scavenging, because scav is a gathering skill. Also, no, it doesn't save your old stat and that's wonderful because otherwise everyone would just switch back and forth and be able to make EVERYTHING with zero penalties, in which case it may as well just be a solo game offline. That said may want to be 100% sure in your skill choices if you plan on putting a lot of time/cred into it.