I'm not familiar with a Poly, but your alignment splines should not be an issue with a new flash hider. If they are not in proper alignment, then you have bigger problems.

Without seeing your muzzle brake, I hesitate to say about grinding flash hole grooves. Too easy to muck things up. As long as your splines are in proper alignment, it is just as simple as buy new flash hider, remove old setup, and put on the new one. Could you supply photos of your rifle for clarity?