Hi Pete! Many of the keyboard shortcuts changed along with the UI/features of Axure RP 9. The current list of shortcuts is available here:
As far as how to navigate nested dynamic panels without that shortcut and without accidentally exiting the dynamic panel, at the top-left of the dynamic panel editor (in the teal colored header) there should be a breadcrumb trail listing the parent panels in the nested dynamic panel setup; you can uses that to navigate to a specific panel within the stack.
Hopefully that helps!