I am replacing my gas range and there is a gas shut off behind the existing stove. I know that I have to turn the gas off prior to disconnecting however because of the wall behind the stove I am only able to turn the shut off valve about a quarter-turn. Will the quarter-turn shut the gas off? Or should I shut off gas at entry to home? Will I then I have to reignite the pilot light on hot water heater and furnace? If I were to "force" the turn off switch into the wall a bit to help complete the turn, would that help?

It would help just as long as the gas is shut completely off

The older gas lines used a ''gas cock'' that only needed a 1/4 turn. They have a cone with a slot in it. Parallel to the pipe, gas can flow. Turn a 1/4 turn, and the slot is cross ways , blocking flow.

Labman-
Thanks as always for your help. The gas line was installed about 12 years ago - not sure if that is considered an older gas line. I wrote in error that I am able to turn the gas cock a quarter turn. It is actually less than that - more like 1/8 turn. After making the 1/8 turn, the burners were still able to be ignited. The turn is limited by the wall though I can see that the wall is slightly pushed in, perhaps to allow for a greater turn.

If that wall is drywall just finish your 1/4 turn and make that dent a little larger. You might try pulling on the pipe to see if there is any play in it, likley there is since someone else turned it on at least once. . You should replace the flexible pipe with a new one that is coated with yellow plastic. Read the attached for more info. Gas Range Purchase information, gas piping, compare prices

A newer line may have a ball valve, still a 1/4 turn, but much less prone to freeze or leak. Yeah, a bigger dent in the drywall behind the range may be as good as anything.