How do I move my incomplete tasks from Sprint n to Sprint n+1, yet preserve the basline for Sprint n

[Abhi Thakur]

If I am moving the incomplete task of Sprint n to Sprint n+1, I am losing the baseline(estimated effort, graph of estimated vs actual) which was created with Sprint n.

Example: At the beginning of the sprint, User Story  8 story point. Has 4 sub-tasks. Each of duration 2days each.

At the end of the sprint, 4 story points are completed. 2 sub-tasks are completed. 2 subtasks are incomplete. Now if I move 2 subtasks to Sprint n+1. I loose the baseline history of Sprint n. 

There should be a way to preserve the history of Sprint n and continue the remaining tasks in Sprint n+1.

Have not able to figure it out yet. Have anyone come accross this issue?

Regards

Abhi