Replacing hose bib

I have a leaky Y-shaped hose bib that I need to replace. It doesn't look that difficult, and I'm willing to tackle it, but I'm up against a brick wall. His name is Joe, and he says I can't do it the way I think I can. This is in the pump house for our well. Also in there is the pressure tank. And a switch on the back wall. I think the switch, if turned off, will make it possible to change out the leaky faucet, by stopping the water from running. He says it will screw up (his technical term) the pump, and thereby destroy the entire system.
I suggested that I turn off the switch, and see if the water stops, but he says the same problems will occur.
I don't see it. The electric goes off occasionally, and I've never had to re-prime the pump or do anything. In fact, it never occured to me that I should.
I can't afford to lose this outlet, it's the way I water the gardens and the livestock. Both would be upset if the water stopped.

Thoughts?

Thanks :)