Sorry ... let me break things down a little bit.
Step one - locate a "likely" E38 ECM at a reasonable price. The photo attached below is a search result from Car-Part.com from Ohio locations - a 2012 Express 2500 6.0L E38 ECM for $60.
This is the same engine that you have.
Step two - read the E38 ECM using HP Tuners (do not license it - just read it)
Step three - determine the OS (12653249 is ideal -12647991 could still work)
This is what I was suggesting - a low cost E38 ECM from your local wrecking yard - and do not license it until you are 100% certain that you will use it.
To answer your question - with the Gen 4 ECMs you cannot change the OS using tuning software. That is why it is crucial to choose wisely from a wrecking yard - such that the calibration in the E38 ECM will work for your conversion project.
The above is depicts the low cost, DIY approach. You could purchase a new E38 ECM, and pay someone to VIN program it to exactly what you need - however with some carefully searching of your local wrecking yard, you could get nearly the same thing - for a fraction of the cost.
Car-Part - 2012 6.0L Express E38 ECM.jpg