Take it to a welding gas place and swap it out. It will be faster than waiting a few days for them to fill it, plus they will deal with the testing. It costs around $15 here for a 5 lb tank, and $20 for a 20 lb tank.
On google maps you can search for welding gas and a bunch of places should come up. Call them and see if they'll swap. The main places around here are Praxair, Airgas, and Central Welding. You'll have the same or similar places in your area.