Change is likely to be permanent. However seen that from S19 the EY15/16 will change back to B777 and EY21/22 goes B787 instead.
given the amount of cuts EY is making, and the fact Jet Airways is starting MAN this winter (don’t forget EY Carries a lot of connecting pax for Jet ex-MAN), I think a downgrade to B787 on one flight has meant MAN gets off incredibly lightly.