The TAX and TT12 are file extensions used by tax preparation software called TurboTax that is available for Mac and Microsoft Windows. You are able to view content of these files in the TurboTax, but there is no reason to convert them, because it is the same file format, only with different file extension that indicates year of tax return.
Convert TurboTax tax return file to TurboTax 2012 tax return file.
We found one software available in the database for this conversion.
Microsoft Windows software - convert tax to tt12 on Windows
A tax preparation software developed by Intuit
Convert from tax file format
The table with program actions contains information about what each program is capable of doing with their files. This may be a good pointer for further searches for certain file conversions and help you find the converter you are looking for if our search result is insufficient for you. However, it is far from perfect and may sometimes show results which are not really usable because of the nature of the software handles the files. So a conversion that appears as possible, just because of matched actions, may in fact not be possible.