早已 and 早就: what's the difference?
早已 and 早就 are different. They can be exchangeable in practice and may or may not have the same meaning. But their grammar structure are different.

Examples-1:

他搬走了.(He moved) (To answer what happened to him?)

他已经搬走了.(Since he moved, he is no longer here)

他早已经搬走了.(1) (Since he moved, he is no longer here. That happened long time ago.)

他早已搬走了.(same as 1)

Examples-2:

他搬走了.(He moved)

他早搬走了.(He moved)(That happened long time ago.)

他早就搬走了.(He moved)(I am sure that happened long time ago.) (How come you do not even know it ??? )

早已 has the basic meaning of 已经. The combination logic is the 早 is used to modify 已经.

早就 has the basic meaning of 早 as adverb to modify the verb 搬走. 就 is to emphasize and to be very sure about it.