Give best fittings a ring. Just google them. Highly recommended from this forum.
They specialise in all things air and air fittings.
Never do anything with high pressure air without the cylinder being fullyl closed, but replacing a gauge should be ok with the right tools as long as its on the output side and not showing contents even with the cylinder closed. If in doubt pop into where you get it filled and they should help.
MAD for it.
Steyr LG 110 Gary Cane, Leupold 2.5-8x36 TMR
Member of the UYK, UYB, UYC, UYH & UYE club.
I would like to agree with you....but then we would both be wrong !!