Situation 1: When a sim is pregnant and has a baby before she is married, the baby will have the last name of the mother. When the mother and father get married after the baby was born, the baby still has the last name of the mother. So you want to change the last name of the baby.
Situatie 2: If you want a Family Tree with the last name like this e.g.: Landgraab I, Landgraab II, Landgraab III,... This is only possible if you change the last name of the next heir.
The Sims Blender
You need EP Pets to play!
Download The Sims Blender here: http://www.simbology.com/smf/index.php/topic,20.0.html
Go to Buy Mode -> Miscelaneous -> Miscelaneous
Buy the tree named The Sims Blender.
Press the tree and choose More... -> Names...
Choose Change Someones First name.
Choose the sim whose name you'd like to change (remember the first name).
Type the last name you want.
Press the tree again and choose More... -> Names.
Choose Copy a First Name to Last Name.
Choose the sim again whose name you'd like to change.
Change the first name again.
Your sim's last name has been changed!