2D Character Control for shmup

Hi everyone, my team and I are currently in development for a new 2d side-scrolling shmup. I have the basic knowledge of using trigger colliders to cause certain events to happen in Unity with C# scripts, but my question is this. How difficult do you think it would be (for boss battles only) to make a centralized trigger that extends the entire height of the game stage connected to the boss and have the player simply change orientation and firing direction when they pass through it. All other keys would remain exactly the same (such as, right arrow will still make the character move right on the screen regardless of direction they are facing).

Really easy.

Make the player ‘model’ a child of the gameObject which you’re actually controlling (with little to no logic on the model itself).
Just rotate the player ‘model’ by ninety degrees (or whatever).
Also change your existing code so it uses the model’s .forward to orient new projectiles.