There may well be a change due to “operational reasons” which is that catch all phrase that explains nothing. If you are indifferent about row 18 as opposed to row 46 I would just forget about it. If for some reason you aren’t together in row 18, then I think your only option is now to check at the airport to try to move seats. If you have a return leg back to the US, you might check at airport or call EI to check your seats on the way back. Just to note that in terms of paying for seat assignments, row 18 is twice the price of row 46, so EI probably think they have done you a favour (assuming you are seated together and have a window/aisle combo you’re happy with).