What to do with the user story points when you find out that the story is of 20 points instead of 8 points? This is not a rare scenario. This happens frequently with teams who have adopted scrum recently and the team members are new to scrum. This can sometimes happen to experienced scrum teams when they encounter a task which is completely new or no one in the team has done such task before. Example could be some research project or developing a Proof of Concept (POC).
According to me there would be two approaches that can be taken depending on the situation:
Option - 1: If the difference is too big then assessment needs to be done for the additional points.
Check if the additional activities or tasks can be done in a new independent story or not. If it can be done perfect; Add a user story for it and renegotiate the scope with the product owner or the stakeholders. In this case the original story points will remain the same.
If the activities or the tasks can't be done in a new independent story and if the the team can't deliver in current sprint talk to the product owner about it and re-prioritize the items in the sprint( apparently renegotiate the scope for the sprint). Here in this
Option - 2: If the difference is small it would be simpler. In this case keep the story points same but attach another card/story to make sure that the additional story points are accounted for and reflected in the sprint burndown chart so that everyone is aware of it.
Last and most importantly bring this issue during the Sprint retrospective, discuss it and identify what lead to incorrect estimation?
This is just one way of thinking. Another approach used by scrum masters or prescribed by Coaches/Trainers is not to change the story points and keep it as it is as the original estimate is part of the history.