I can see three (not two) degrees of difficulty and three degrees of fairness here:
a) modifying past service for current workers: most difficult and most unfair
b) modifying future service for current workers: somewhat difficult and debatable as to fairness depending on the promise that was made to current workers at time of hire
c) modifying future service for future workers: not difficult and can be done in a fair way as long as future cost is transparent as part of total renumeration.