Back out the adjusting screws on the gauges before opening the tank valve for safeties sake. This should be done all the time, but much more so when using old gauges just in case the diaphragm is bad. If you don't hear any leaks, then slowly turn in the regulator adjustment screw and watch the low pressure gauge to see how it comes up, if still no leaks, you should be good to go. All the torch O-rings can be easily replaced and most LWS will have them, but they aren't cheap. I think the torch head o-rings that fits between the head and mixing barrel was almost $5 for my Victor torch. Then there is the high pressure O2 for the cutting air valve seat that might be bad but that is all there is in the cutting head. I rather doubt that any of the components will need anything other than possibly new hoses from dry rot.