When I opened my brothers to solder/tape the cord back together, I followed some
instructions on instructables where you start with a needle nose pliers to push apart the outer shell at pop up cord winding ears.
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and with a liberal use of a flat head screwdriver. Once I got it started with the pliers, I inserted and moved a flat head screwdriver along the groove along the skinny sides of the shell, gradually popping them apart. It will leave some marks on the plastic but not nearly as much damage as using a flat head alone (or more destructive methods) to try to separate the halves.
hope this helps