Do you mean replace the existing floor plan file with a new one?
If so, here's a copy of my response to another thread that asked the same:
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Different ideas have been thrown around for this. If you have any good suggestions, we'd love to hear any ideas on how to improve this.
The reason for deleting and re-adding the floor plan is due to the final image used for mapping. Original floor plan was added with height / width dimensions, these dimensions are converted to a scheme of x,y coordinates on a grid. Any devices added also have x,y coordinates matching this image's grid. The x,y coordinate scheme as you can tell, is the sole reason replacing the floor plan won't work in the current implementation. There's no guarantee that the replacement image will have the same exact scaling and cropping of white space for x,y coordinates to be consistent with the already learned coordinates.
Ultimately, the need to delete and re-add floor plans for replacement forces users to redo some work, but increases the chance that they'll have accurate placement of devices on floor plans in VisualRF.
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The original thread that this response came from:
http://community.arubanetworks.com/t5/AirWave/Any-way-to-replace-floorplan/m-p/45616#M853