For non-significant changes which do not affect the business purpose of travel, you do not need to re-create a Travel Request. Such cases include a change on the departure date or return date by a day or so. When creating an Expense Report from an approved Request, all fields on the report header that come from the Request may be changed to reflect the updated information.
For significant changes that affect the purpose of travel, you will need to create a new Travel Request and inactivate the old Travel Request. Such cases include a cancellation of the original trip and scheduling a new trip instead due to an unexpected business related event.