Assuming you are in a region where you are allowed to redownload your past music purchases, delete the current copies from your iTunes library, go to the iTunes Store home page, click the Purchased link from the Quick Links section in the right-hand column, then select Music and Not on this computer. You should find download links for your tracks there.
If the problem persists, or that facility is not yet available in your region, contact the iTunes Store support staff through the report a problem links in your account history, or via Contact Support.
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