The fundamental source of the problem, as you have confirmed and we have already told you at least three times, is that you must use Disc-At-Once and not Track-At-Once to overburn.
If your question is why Nero defaults change when upgrading, I don't have a good answer other than that Ahead doesn't bother to maintain all settings during the actual upgrade process. Don't feel bad; you aren't alone in seeing this phenomenon. But once you restore all your preferred settings after the upgrade, they should stick.
Nevertheless, it is always a good idea to review all settings before any burn to catch just such problems. I've seen many people swear up and down that they have everything configured properly, and yet their problems persist. Only after much pulling of hair and gnashing of teeth are we able to convince them to actually look at the settings, at which point they discover something isn't what they thought it was.
This isn't a knock against you, just a general exhortation to everyone that when things aren't working as expected, double-check and verify actual settings rather than assume nothing has changed since the last time you used a piece of software.