- What's the difference between the two?
- If there are no notable differences, what was the motivation behind the change?
A Route Army was a "local" army defending a relatively small area such as a city, or a road leading to it, hence the name "route" army.
During the second Sino-Japanese War (which "morphed" into World War II), fighting took place on a larger scale, and over larger areas than previously, so the Chinese adopted a Soviet-style military organization covering "regions" or "military districts" defended by "Army Groups."
Some of the larger route armies directly became army groups, but most of them were formed by the merger of two (or more) "route armies," e.g. the ones defending a city and the surrounding area.