Well actually it didn,t change if you had it on the page previously...the old nav was the bloaty radcontrols version. What you see there is the new clean markup version with which you can actually tweak the markup.
So the upgrade to 6.3 didn't delete that widget from your page and replace it with the new widget...sorry, but just didn't happen. Maybe you dleted it and readded the 'new' one, or someone else did.
Nothing happened to the old widget, it's still there for legecy puroposes, however it's depreciated. There was PLENTY of warning well before 6.3 on the roadmap and IN the release notes.
What is borked though is that the upgrade never installed the widget templates for your dropdown. I've seen this happen on about 3 sites. Support can help you restore them...I used JustDecompile and copypasted the markup into a new template.