Hi pkw1966, I have never heard of them assigning a guarantee this early.
So that is good news you have your assignment so early, it gives you time to fix the bad news of this cabin. You do not want in the main aft or forward of this size ship, not to mention in the gulf in September.
You are no where near penalty, I would call the PVP and tell them it is unacceptable. Even if you have to re-negotiate an assignment for $20 more, I would do it.
That cabin is certainly inconvenient, can be bouncy, and noisy as well as Dave noted TN mentioned. I have been on Holiday's sister Celebration in the main front during rough seas..Never Again!
Hope this helps as an honest opinion to your question, and again the good news is you have plenty of time to fix it
Best to you,