POUR ½ cup of grape juice into medium saucepan; sprinkle with gelatin and let stand for 1 minute.
PLACE over medium-low heat, stirring, until gelatin is completely dissolved. Add sugar and stir again until dissolved. Remove from heat and stir in remaining 1½ cups grape juice.
SET saucepan in larger bowl of ice and let stand, stirring every few minutes, until mixture thickens and begins to set, 12 to 15 minutes. (Or place in refrigerator, checking occasionally.)
DIVIDE halved grapes among four (8-ounce) glasses or bowls. Pour some of grape gelatin into each.
COVER with plastic wrap and refrigerate for about 2 hours or until completely set.
WHISK yogurt in small bowl until smooth, then mound on top of jellies.
*Or substitute 6 packets or ¼ cup Splenda.