I make premade sims and townies over all the time. With the premade sims I try to be very subtle about it, when it comes to their features, because I do think they made them with a lot of character. But some of the kids just do not age up right. Alexander Goth is a prime example for that. My last case of extreme makeover was Evie though, the toddler from the Delgato family in Brindelton Bay. One of the kids in the family I was playing made good friends with her and I thought it would be cute if they would fall in love as teenagers. A day after he aged up I invited her over and was pretty shocked.
https://i.imgur.com/EzAfiZw.jpg
So I had to help the girl out.
https://i.imgur.com/UZMe4Dr.jpg
Now they moved in together and got married, all thanks to CAS surgery. :)