If history is anything to go by, we would see a softening of the Khmer Rouge stance as ideologues are gradually pushed out by Moscow-trained bureaucrats, leading to a more settled pattern of life and fewer atrocities. Of course, Pol Pot was a monster even by Stalinist standards, but if either the USSR or China have a reason to prop him up the diplomats might just hold their noses and smile. There would have to be resettlement in the cities (though there would be some kind of ideological justification for that). Other than that, Campuchea would be dirt poor, economically crippled and in deep societal shock denial. In a best-case scenario, expect a gradual admittance of 'overshooting the mark' in 'ideologically misguided' purges of foreign influence during the 'years of the personality cult'. Worst case, of course, is Pol Pot living to a ripe old age and passing power on to a designated successor.
And to think that we have, of all the people, the Vietnamese to thank for ending that...