fully opening. If you haven't turned off your main in a long time, there can be an issue with the valve. It's not anything too terrible, and if it's bad it's better to know now than in a real emergency. Worst case here is that the main doesn't fully close and when you open the shower valve to replace the part you have a stream of water into the shower drain. A main valve that doesn't ever close becomes much more of a problem when you're working with the plumbing in an area that's not waterproof with a drain. Under cabinets or behind refrigerators gets much more problematic with a main that won't close than a shower valve.

I recommend if you have to change the main, always replace with a quarter turn ball valve.

Or, you can turn it off at the street and not mess with your house main.