They should move. Perhaps it would soften the blow if they were also told platitudes about fault and blame, but realistically (a) the problem is not going to be solved in time, and (b) the plight of a small number of people is not going to move the needle in regards to the willingness of the world to address the issue. People who still think that the damage can be averted entirely are just as deluded as people who think that sea level rise is an existential threat to human civilization.