The most obvious change by having the Regiment and Ethnic Diversity Modpack enabled is the rescaling of unit sizes so the difference between soldiers and armored units is more obvious. It's not an extreme change, but rather one that adds to the graphical variety of the game. The mod also adds new unit models and as well as new formations to reflect the change in scale. Find it at the Steam Workshop.
For a rescaling that's even more dramatic, stack this mod on top of the R.E.D. Modpack to boost the number of units in each formation. Be sure to install Xtreme after the original Modpack for the correct display of units. Available here.
Inspired by the visual update provided by the R.E.D. Modpack, Ethnic Units adds a new level of diversity to a long list of units in Civ V. And even two years after its initial release, it's still being updated. The latest version from June added new takes on Polynesian swordsmen and French cavalry.