You are correct, using a chisel blade to crack open the shoulder joint for a lower arms swap can cause too much damage to the part.
Using an X-acto blade, score along the seam until the blade can get halfway deep, then get a small flat head screwdriver and using its edge, twist to pry the seam open. In some cases the seam will come apart easily just using the X-acto blade. Take care not to crack or break the shoulder joint completely off. Swap the lower arms out and try out the new set of lower arms you wish to use. It sometimes may require some sanding or cutting for them to fit correctly to function but make sure that the arms fit right before gluing them.
I hope that helps...
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