If I buy a title on disc through retail and later buy the same title through Games on Demand (XBOX Live), will the save games carry over?
I'm thinking about Earth Defence Force 2017 in particular.
Everything I have read (nothing official, but plenty of posts elsewhere) suggests that you should be able to use your saves with a Games on Demand title ... saved games are independent of the method used to play them, so just as you could play a game from a disc, later install it to your hard drive, and use the same saves in both places, you can purchase it through Games on Demand (installing it to your hard drive) and use the same save.
The biggest issue will probably be where the saves are stored. Some services, such as Steam I believe, will store them in a different folder than normal (in Steam's case it is in an actual Steam folder for all saves.)