First off I don't recommend anyone attempt to take apart and repair a regulator because it's just plain dangerous considering the pressures you are dealing with.
, now having said that if yours seems to be working ok this time then it will probably be safe enough. If the regulator fits the tank you are using then there is no reason you cannot continue to use it for either gas it is just that each will flow at a sighty different rate for a given pressure setting. This really does not matter much and instead of using a PSI setting the better way to adjust the gauge would be to turn down the gas flow while welding (on a piece of scrap!) until you start to get porosity in the weld, you will easily see when the gas gets too low, and then turn it back up until the weld is sound again then add a couple of PSI. Trying to use a certain PSI setting can waste a lot of your gas since it is the flow you are concerned about not the pressure. At any given pressure the flow could be quite a bit higher or lower from one welder to the next depending on equipment being used, what pressure setting gives the right flow for one welder may or may not work OK for another one. Setting the gas flow using the method I described will give you the correct flow no matter which gas you may be using and it very well could save you a lot of wasted gas. Having the gas flow set too low obviously can cause a lot of problems and too high, while it will not hurt the weld, (within reason of course) can waste a LOT of gas.