Item IDs are typically not preserved during migration, and it is recommended to not use them as a form of identification for Items in a SharePoint list or library. However, sometimes it is important to preserve these values for reference cases. To do so, use the following process:
1.Copy source list to the target without any content.
2.Go to the newly created target list and add a field to be mapped to (Legacy ID in this example).
3.Refresh the target site in the tool.
4.Copy all items from the source list to the target list, then find the newly created field on the mapping screen.
5.Choose the downward facing arrow for the added field (Legacy ID), enable hidden columns, then choose the ID field to map from, then click Finish in the wizard.
6.Post-migration, review the list in the target to verify that IDs were preserved to new column.