It seems that both .tax2015 and .tax files are used in TurboTax, albeit in different versions of the program. It is unlikely that you can directly convert tax2015 to tax or vice, at best you will be able to import the data and produce some exchange output format from it, but not convert one form to another.
This conversion entry was verified by us and conversion was flagged as wrong or not possible.
Usually this happens when the conversion would try to convert different types of data between each other, such as audio to image, or it is not possible because incompatibility between programs, caused by closed file formats, very limited support etc.
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