Do not thing abstract.
If the same concrete term applies to both effects, it can be the same spell. "soldiers" is such a term, leaving the details of weapons and banner to casting time. But you'd do quite some semantic gymnastics to find a common term that encompasses both soldiers, horses and a bridge.
So a spell that creates an illusion of "a charging host of phantom knights" would be specific enough as it states the type of unit, their actions, and number? But is general enough that you can specify the details of their armour (though, clearly its heavy armour as they are knights), nation, and general appearance.
Some of the differences between "change" and "control" seem a bit hard to understand for me - especially when dealing with elemental items, like fire and wind. Sometimes by "controlling" fire, you can change what it is doing, how it acts, or where it is. Will this be specifically up to the details of the spell to determine if it's a "Control" or "Change" issue for the GM who reads it?
I think its possibly because as far as elements like water, wind, and fire go - they don't have a set "shape", so changing them can be achieved through controlling the actions they take.
Though it's not always that hard - water to ice is a change (state), where as direction the wind is blowing is. . . change? control? Extinguishing a fire or flaring it up?