of course, all these modes and settings depend on each camera spec, but if you have a better then 'plain vanilla' camera it normally would allow for half-pressing 'the shoot' button to take focus without taking the picture yet (it will show you some kind of brackets around the area of focus; and that is usually in the center area). if you then move the frame to put your subject slightly off-center (without releasing the button) - your focusing will remain on the original spot. this way you can be assured that smth that is not in the center is still sharp. then you press it all the way to take a pic. of course, some areas may not come out sharp...(depending on how close your in-focus area is, how far your depth-of-field is..,etc)
but this 're-framing' is normally just the function of auto-focus and should not require you to change modes.
