I am assuming you mean an array of DINT with 256 elements?
If the data in the source tag already exists example x_256[256]:
Go to the Tag Edit screen, and right click the array, then select copy.
Go to the new tag (bottom of the list), right click and select paste. The new tag will be created based on the name of the original one appended with a "1". It will contain the same values as the original. Then you can rename it as desired. Edit: You may have to blow away the existing tag first...then rename the new one to replace it...
Another method would be to use RS Emulate 5000 and do it with the virtual PLC COP instruction.
Hope this helps,
Paul