Mmmm interesting question … how to convert track presets en masse to use a different plugin.
As far as I know there is no documented way to do this other than manually, track preset by painful track preset.
In your previous attempts, it looks like you only changed the display names. Changing the name of the DLL is unlikely to be successful either because VSTs have a unique identifier embedded in the file. Perhaps you could create an instrument track with only Kontakt 5 loaded, create another one with only Kontakt 6 loaded, export them both as track presets and compare the files? If you’re of the programming sort, you might then be able to script a conversion utility.
On Windows, the files are located in %APPDATA%\Steinberg\Track Presets<type>, where is one of Audio, Instrument, Midi, Multi or Sampler, depending on the type of track preset. The files have an extension of “.trackpreset” but are XML text files. I’m guessing the plugin is identified by a unique ID which it gets when it’s scanned; on this machine, for Kontakt 5 that appears to be:
<member name="Plugin UID">
<string name="GUID" value="5653544E694F356B6F6E74616B742035" wide="true"/>
<string name="Original Plugin Name" value="Kontakt 5" wide="true"/>
… though I could be wrong. If you can find what the equivalent GUID is for Kontakt 6, you could try putting that there instead and see what happens.