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I had to do this myself as well and had the same problem as you.
Firstly, you need to empty your templates_c folder for one thing. If that doesn't work, in the 'Rebuild Thumbnails' section of the module there is an option to force re-creation of a thumb even if it exists. This should ensure it overwrites the old one. Do that, then empty the templates_c folder again and you should be OK.