Hi Marc
Your Original Estimate is locked down at the point you start your sprint - until that point any changes on scope automatically update the Original Estimate. Once you start the sprint then changes to Estimate are recorded but do not alter the Original Estimate as this was locked down but do impact the scope to be completed in the sprint. When you view the burndown chart for instance you will see these changes reflected in the red line showing the remaining work. Scope added to the sprint will cause the redline to go up, whilst scope removed will cause it to drop. However, the grey line of the predicted completion which was calculated using your original estimate will not alter.
The reasoning behind this is that a team is committing to deliver a certain amount of scope in the Sprint and this is reflected in the Original Estimate - it is expected that changes in scope during a sprint will be minimal and most frequently be the swapping of similar size stories (but only ones that have not yet been started) where a new dependency is identified.
I hope this helps explain the logic behind what you are seeing.
Phill
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