This is not an easy task, depending on the transaction or dependent record history, this can be a huge undertaking. Some key things to consider:
What is typical open transaction volume? do you plan to replace all open line items?
Are there open intercompany transactions? (this one creates a huge headache)
How will you assign serial numbers to existing inventory? Ultimately you need to adjust out and adjust back in as a new item, so be sure to establish timeline for this.
Hours of operation? Can this be done outside business hours.
Any 3rd party integrations with item mapping?
Historical transactions should remain with the old item record, so you will want to establish a nomenclature for deprecated item that does not completely kill the usability of historical reporting