I don't think your tax increasing is related to changing your Cost Basis from $0 to the correct amount. When you have a Cost Basis other than $0, your gains should go down, so if anything your tax should go down also. For example, if your cost basis is $0 and proceeds are $10, you have a gain of $10. If you have a cost basis of $5, and your proceeds are $10, you only have a gain of $5.
Since you imported this info, you should delete your import and combine all these types of transactions into a one-line manual entry, and avoid all the editing. Here's How to Enter a Summary in Lieu of Individual Transactions. Editing imported transactions can be problematic.
If you're using TurboTax Online, clear your Cache and Cookies after deleting the import and re-entering a manual summary.
@ragsns
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