Sure, change everything you like as long as (i) the players are aware that your game does not follow the cannon after X date, and (ii) that your changes are internally consistent.
The former is easy but can get players' back up when an NPC acts in a different way that they imagined said NPC would act. But this is just expectation and can easily be managed.
The latter is harder to achieve but not impossible. Sudden changes in NPCs' motivation or thoughts should be taken with care so as to maintain realism within the game world. A lot of Song Of Ice And Fire is based on some character being unaware of something happening or lacking knowledge pertaining to a certain situation. If those change, then the characters could react differently as to how they do.