Hi James,
You can convert to the latest Mx6 version (6.10.7), than convert to Mx7.2 by following this how-to.
Edit: Converting to 6.10.7 does not work, try another (6.10) version. I tried 6.10.4 to 7.2. When opening the 6.10.4 project in Mx 7.2 you get the dialog if you want to convert, which is correct.
If want to migrate an app from 6.10.7 to 7.2.0 you get an error, not the dialog, This looks like a Mendix bug.