I live in a trailer park and each unit has a main shut off before the unit's water meter. However, the park only has one street shut off for the whole park. I watched a plumber replace a unit's main shutoff (a PVC one) with a brace gate valve without turning the water off but am a little unsure how he did it. I've got to replace my unit's main shut. Has anyone had experience doing it?

I think he put threaded male PVC fittings in the brass valve, opened the valve, cut the supply line and put PVC glue that can be used on wet surfaces on the pipe and then pushed the "open valve" on the pipe. Let it set for a bit and then closed the valve to shut the flowing water off. If I remember correctly, he then reconnected the new valve to the water meter.

Has anyone had experience with this situation?

cut the supply line and use a sharkbite ball valve in the open position.
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What kind of pipe is it? Please post a picture

Why do you need to do this? Is the old valve broken?

Another option is to add a 2nd valve downstream of the first valve. It would be easier than what you are suggesting.

That glue only works on "wet" pipes, NOT "flowing ones". I am not sure how he actually did it, but someone got very wet in the process of the main water line has "real" pressure. (Try doing it in the middle of winter on a 2" line with a roof 4' above you and you have to take the new valve apart to screw it on. Now that is WET.)

The pipe is a PVC water line. The old valve does not shut off all the way anymore - thus a water leak when trying to repair or insert new plumbing to the house side of the valve.