For an item that has site and warehouse activated and no tracking dimensions (only Site is financial), it's required to activate serial numbers. No product dimensions. No WHS.
All transactions for the items has been completed (purchased or sold status) and inventory set to zero.
Items cost model is Moving average. So, Inventory closing cannot be performed.
When trying to change the Tracking dimension group to serial physically activated. The following error is shown:
"The new dimension group cannot be assigned. Physical inventory transactions might exist and/or financial inventory transactions are not fully settled, while 'Activate closing of non-financial transfers' is set to Yes".

Theoretically, when for an item, all Inventory on-hand records, and then grouped by Site + Product dimensions (configuration, size, color, etc) and Financial storage dimensions (Site in your case), the following is true:

(inventSum.PostedQty - inventSum.Deducted + inventSum.Received) >= 0

Then, you should be able change the Tracking dimension group.