What happens to processed expenditure items when I add a project or task burden schedule override?

If you change the burden schedule for a lowest-level task with processed expenditure items, then the expenditure items aren't automatically marked for reprocessing.

Only new expenditure items charged to the task use the new burden schedule. You can mark the expenditure items for recalculation and reprocess using the new burden schedule assigned to the task. You can manually adjust the expenditure items to recalculate the burden cost amounts by using the new burden schedule assigned to the task and then reprocessing the expenditure items.