I found a thread on Boardgamegeek.com that explains some differences:
https://boardgamegeek.com/thread/659828/difference-between-hail-caesar-and-warmaster-ancieAlthough built on the same basic gaming engine, there are some pretty significant differences:
1. In WMA, most units consist of three stands. Units in HC come in varying sizes: standard, large, small, and tiny. Unit frontage is a key factor, and most units are at least two ranks deep.
2. There is a wider range of troop types available in HC. There is also a much greater variety of possible unit formations.
3. In WMA, each unit may be given up to three commands per turn, with each command and die roll being made in succession. In HC, the unit's orders must be stated out loud, then a die roll determines whether the unit can make zero, one, two, or three moves to complete its orders. Move distances are also a bit smaller in HC.
4. In WMA, each unit has a single Attack value. In HC, each unit has two separate values for ranged combat (Short and Long), and two separate values for hand-to-hand combat (initial Clash and Sustained fighting).
5. There is no Armour Save in HC, although the Morale Save serves a similar function. In HC, each unit also has a Stamina value - if a unit takes more hits than this value, it becomes Shaken and suffers various penalties. Under certain circumstances, units being attacked in ranged or hand-to-hand combat also must take a Break test to see if they hold, give ground, or break and run away.
6. HC also includes a number of useful optional rules to add historical characteristics and flavor to various units.
7. In WMA, armies tend to be built to standard point values. HC is much more oriented toward scenario play, with little emphasis on points-based armies. The HC rulebook includes seven complete sample scenarios.
There is more, but hopefully this should give you an idea of some of the differences between the two. Hail Caesar is quite flexible in regard to unit size and basing, so if you choose to pick it up, you shouldn't have any trouble adapting your WMA armies.