I find myself usually just restarting the family from the existing save. I did this when Parenthood dropped, for instance. This time, instead of abandoning my current save, I have created what I'm calling a Flashback save. My Global Superstar has grown into a YA, he earned this status as a Teen. Has married, has graduated Uni with a distinguished degree, and is the father of six children. He's also winning Best Song Accolades every week. I will NOT abandon this save. But I do wish to see this Sim in high school. Hence, the compromise is the Flashback save. I will marry the two saves together at some point and then write up in flashback form, Erik's own experience in the upper level of his education. I think this part will begin when his eldest child, Jamison, is about to begin high school. Unless, of course, this save turns out better in many ways than the existing one. Time will tell. And, yes, I am the GalacticGal referenced above. ;)
