What to change when migrating designs from Altera DE2 to DE2-115?
You need to find the pin mappings in the project and update them so that you are using the "new" pins in your new project. Quartus has a GUI for this sort of thing.

I'm surprised it builds without that, but it may have just selected a random set of pins for you.

You'll need to get the pinout for the new board and decide for yourself which pins youwant to connect to which of your design signals.

edit:
A link to the pin mappings is available at the bottom of this page (the QSF file): http://www.altera.com/education/univ/materials/boards/de2-115/unv-de2-115-board.html