Argon mixes are at a much higher pressure so the regulator needs to be rated for it. A reg for CO2 may not be rated for 300bar as it only normally runs at 60 bar.
Mine's rated for 300 bar so I guess I'd be ok with this if I wanted to swap to an argon mix at some point. Mind you, a new SWP reg from ebay is only about 7 quid dearer.......