Do not overwrite the figures. Register each new deviation as a separate item, an expected addition, and use transactions when you move money between production codes. That way you have the history in one place.
How to do it
When a new deviation occurs, create a new expected addition on the production code with + Add → Expected Addition, instead of changing the amount on an existing one.
In the description, write what the deviation concerns and why it was entered.
If you are going to move money between production codes, do it with + Transaction in Prod. codes. The transaction shows both which code the money came from and which it went to, and you can find it again in the Transactions tab on the production code.
If you want to see what has happened over time, click History at the top right of the production code.
Good to know
Each item becomes a trail you can go back to. A figure that has been overwritten does not tell you why it changed.
Transactions always sum to zero on the project.
A transaction changes the revised budget. If you want the final forecast to change, the planned costs must also be moved.