I have a problem with this post. You are assuming that old posts performed worse than newer posts because of some magical change to the Facebook algorithm. This is very weak science. You would be better off posting new content today, then measuring how often they get displayed to your friends’ feed depending on the type of content.
Also, it is worth noting that Facebook practices continuous deployment which means that they make hundreds of small changes to their algorithm every month. The idea that Facebook would “flip the switch” on a huge change in a single day goes against their methodology and is extremely unlikely.