This documentation explains how to split a story when it is unfinished in an iteration
Manage Unfinished Work | CA Agile Central Help
But it is missing a step
For the Historical story it should also advice users to remove the the story from its parent
further down on the page it says "If you do not remove the historical story from the epic parent, that epic's scope will be artificially inflated. The epic's burnup will never achieve 100% acceptance due to the unfinished work in the list of child user stories.", which supports this step. But it should be explicitly listed with the other steps
Thanks for finding this great way to improve our Help! I'm working with our User Learning team to make the change.