Generally, if the bars you're going to, IE: Buckhorns, are smaller or shorter then what you have NOW, then you'll be fine. You may have to creatively hid some extra wiring, or reroute a clutch cable to look better, but it should work.
If you're going to a taller bar, or larger, then it becomes a task and you need to be a little more creative.
My Dyna had buckhorns, and I went to 10" apes, but had to replace the brake line only. That's the limit.... anything more, and it's new wires, clutch cable and throttle cables.
The new bars have to have dimples in them as well, to clear for the wiring where the controls clamp on.