There is no clear rule on whether to change an engagement or a marriage, depending on local customs and parents' wishes. In places like Fujian or Zhejiang, parents of both parties will give the couple a red envelope when they get engaged, and the couple will change their mouth when they get engaged. In some northern cities, the change is made on the wedding day after parents give red envelopes.
In Fujian or Zhejiang and other places, people will pay more attention to the engagement ceremony, but think that the wedding is just a form and there is no need to wait until that time to change, at the time of engagement, both parents will give the new couple to prepare to change the red envelope, the new couple received the red envelope, they changed their parents, and received the red envelope is also on behalf of the family. Therefore, in some areas, it is customary to change the name of parents after engagement.
Comparatively speaking, the custom in some northern cities is that on the wedding day, the couple will serve tea to both parents, and after the parents give red envelopes, they will change their name to "Mom and dad".
In fact, the wedding customs are different, so the habit of changing the name of parents is also different, but one thing is the same is that on the day of the change fee, it is necessary to formally change the name of "mom and dad".