Garmin doesn't publish any change history for map updates so it's not possible to easily see the differences from one version to another [except for major ones of course].
Really you don't need to keep an outdated CN as the only option if that's what you mean, in that you're saying you have to use the CX Chicago 2014.10 with the CN NA 2014.10? Try using the full CX 2014.10 with the CN 2014.20 with only the one you're using selected. If you do happen to get the conflict problem reported by coastline2001 regardless of selection choice then simply use the solution he proposed in Post #29 and have the full CX Chicago 2014.10 and the CN NA 2014.20 on separate cards and switch depending on where you are, in Chicago or elsewhere. Otherwise match the versions as you said, both 2014.10, in which case you can use just the small MMR only Chicago CX.