Changing the reference designator for a part in PCB123 is a simple process. From either the layout or schematic, simply right click on the component you wish to change and select "rename refdes" in the context menu. From there just type in the new letter designation you wish to use. The software will the numerically assign a 1 to the component and incrementing new instances of that part from then on out. Please note that this will not update the other like components that you have already placed, nor is there currently any means to autoupdate or group update components that you have already placed in your design (an enhancement request has been submitted for this feature).
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