Changing water valve

I want to change the valve on the water supply line to a ball valve. There is not alot of room in the existing opening probably about 12in square plastic panel anyway the valve thats in there is soldered and I cant move the pipes up or down in order to heat the valve up to remove it. I was considering cutting a small section out and using a union on both ends of the valve. I know this is a vague description but would like some opinions please. This is 1/2 in line. Thanks

How about using compression fittings?

Rather than a union (which would work), you could use a repair coupling. This looks like a 'normal' coupling, but does not have a stop in the middle. This is so you can slide it down the pipe, reinsert the new piece, then slide it back over the end of the other piece, then solder it. It allows you to insert a pipe with no way to spread it apart to insert the normal coupling with a stop in it.