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Magento doesnât have a way of down grading built in. To move to an older version there is one realistic option which would be to rebuilt the site on that version. Any thing else may result in long term database issues.
Tbh, It is much more likely that fixing the existing module to work on 1.7 is going to be less time consuming than switching to an older version.
I agree that creating a new installation would be the least painstaking way to solve the problem. And it won’t be that difficult if you consider everything properly.
Here is a detailed instruction on moving from any shopping cart to Magento: http://bit.ly/WkTKl8
It’s perfectly applicable to switching to an earlier Magento version, too.