This conversion seems to be related to some transfer of various of editions of tax return files, or at least some of their parts in TurboTax or similar accounting / tax preparation software. Perhaps not a straightforward conversion, but doable in the original software, although data in tax return files are related to a single year only.
Convert TurboTax 2014 tax return file to TurboTax 2014 tax return file.
We found 2 software records in the database related to .tt14 to .tax14 file format conversion.
Microsoft Windows software - convert tt14 to tax14 on Windows
A tax preparation software developed by Intuit
Convert from tt14 file format
The tables with software actions are a good pointer for what a certain program does with various file types and often may give users a good hint how to perform certain file conversion, for example the above-mentioned tt14 to tax14. However, it is not perfect and sometimes can show results which are not really usable because of how certain programs work with files and the possible conversion is thus actually not possible at all.
Apple macOS / Mac OS X software - convert tt14 to tax14 on OS X
Popular tax preparation software