Modify of SLC-EZE r/t on skymiles?
 

We have r/t to EZE on Delta with skymiles. Would like to modify our return flight for a shorter stay. Would like to possibly us Argentina or GOL for the return flight. Concerned that changing the return leg also effects the first leg and right now our BE tikts are only 100k. As of now the flights on Delta are all "highest" level. Any comments would be appreciated !) Is this even possible with affecting the whole flight? 2) if possible suggestions as to the most likely routing and 3) is KAL available return route on its flight from SAO to LAX. Appreciate any info at this point.