No, it is not possible to change the item ID of existing items in Portal for ArcGIS. A new item ID is assigned when a new item is created. An item ID is a unique identification for a single content item stored, managed, and accessed in Portal for ArcGIS. This item can be a file, hosted layer, or web map.
In some instances, it may be necessary to use an existing item's item ID in a new Portal for ArcGIS deployment as existing services and scripts may be dependent on the item ID. Depending on the scenario, use one of the following workarounds to replicate or restore an item: