The Orthodox patriarchs have no authority to change the date of Easter (Pascha).
They are not autocrats who can act by themselves.
The Orthodox faithful would not tolerate such imperial actions. Only an ecumenical council can change the declaration of an ecumenical council (Nicea).
So how did the current split happen among the Orthodox between the old calenderists and the churches that switched to the revised Julian calender? I'm pretty sure they didn't hold an ecumenical council to decide which calender they were going to use.