I am sure you have both found the answer by now, but no, you can not change, rewrite, or any of the things asked above if the product was sold or given to you as PLR, unless stated differently inside your license, you only get to use it as MMR etc. If it was an MMR etc product to begin with, then you definatly can not edit or change the main product.
I always look for PLR when I can, but some MRR or resale products can be very valuable also. Use them as lead ins, mailing list subscribes, bonuses, or in a bundle.
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