most places will either cut the valve stem or gash the tyre to let the pressure out.
if they sliced the tyre they may not need to replace the valve.
but when I've changed Tyres on my own bikes in the past I never used to replace them, just let the pressure out by sticking a biro tube into the valve. if they were perished/damaged I would do so.
but taking to a tyre place I make sure I watch em, and they change it!
technically the bike is your property and they have no right to not let you watch, if they say H&S issues...ect say how are the salesmen allowed into workshop then?
surprise surprise when I went in to watch one service done they took an awful lot longer than they did for exactly the same job the time before! funny that!
I never will take the bike to a stealer to have work done, find a local bike mechanic tyre shop and give him the trade!
ps sorry about the rant... SWMBO wants to go shopping and I really don't want to go!