Please let me add some comments.
I took the concept of paper-scissors-stone (square-triangle-circle or "Patton's Push-Rommel's Revenge-Alexander's Approach", whatever you like most), since it is the most easy and understandeable concept and the first one which holds all three: a draw, a benefit and a malus.
I think, Rhialto got me wrong when he states that he either would have to change "every 50 turns" or just "could change only every 50 turns".
The 50 turns just were an example of mine.
In principle, you could change every turn, but it would not benefit you, as I refined the model with a certain delay for your troops to adopt the new tactics (let's say 2 or 3 turns).
Those different tactics should add a little modification and therefore spice to the game without making it pure luck. I did some (very raw) maths and it seems to be a difference of ~ 8%, but this would have to be confirmed.
I calculated with A=4, D=4, T=0.1, H = 5
A = attack, D = defense, T = terrain, H = hitpoints
8% seems to be a value which you may identify after a while, but which doesn't spoil the "normal" combat results. Even more, it will be a negative effect only for 1/3 of your contacts. For 1/3 of your contacts, you will gain a benefit, for the last 1/3 it will be a draw, so there nothing changes.
In total this means:
For a single battle, there will be no significant change. But, for a greater number of battles, you may identify a certain pattern. In that case, you may decide to either switch your tactices or you rely on the soon to be reached mastership, which will cover your slightly worse chances.
With very little effort, the AI can do as well.
Of course this means that they can become masters as well. Under the assumption that they have the "better" tactics in relation to yours, you still suffer those 8% malus.
It could happen, though, that they are confronted with others as well, and have go give up that tactics for the sake of other battles to be fought.
Well, I guess, I have explained it over and over again.
In combination with espionage and maybe a modifier for the switching time (Sun Tzu or other improvements) I see more options for the player come up. But this is something, everybody may decide for himself.