I have recent experience with a guarantee becoming available. 2 months ago, our popular May 2025 transatlantic PH2 guaranty showed availability which I noticed on a weekend. I emailed my TA who promptly answered back that she would take care of it on the following Monday morning. We obtained the available cabin without any problems. There was no charge or change in fare. A couple of weeks later, a better cabin (in my opinion, of course) became available. I emailed my TA once more and we were moved to this newly available cabin, again with no charge. Since it was still a PH2, there was no change in fare either.