They are two different charms.She has not wiped herparents'memories(as she later does to Dolohov and Rowle),she has bewitched them to make believe that are different people.
Why 19 years later?
Of all the time in the Harry’s life that you could have chosen to set the epilogue, why 19 years later?
“I didn’t want some people to have children too young because I don’t think that’s good,” Rowling said. “So 19 years was just enough time for the next generation to have reached the point I wanted them to reach when the Hogwarts Express is departing.”
There is no significance to the number, no magical explanation. At the Battle of Hogwarts Ron and Hermione are 18 and Harry is 17, and Rowling said she wanted them to have some peaceful time before they started having children.
“I don’t want to encourage teenage pregnancies,” Rowling said, laughing. “It couldn’t be much earlier than 19.”