If you choose to upgrade , then I would suggest changing to another flight for DXB-MAN which does have 1st class rather than spend that amount just upgrading 1 leg

As far as what happens if they get rid of 1st on one of your flights; if it is a change that happens in advance, then contact EK and get rebooked back in business and get a refund; if 1 leg ends up without 1st then you may find yourself in business class on that leg but having paid the same