If you get a different confirmed date after migration then either (a) the previous confirmed date was incorrect and not supported by actual inventory availability or (b) the inventory availability or ATP configuration in SAP is inaccurate (c) orders were entered in a different sequence and a wrong order got "dibs" on the inventory first.
If you correctly migrate the inventory data, the POs and whatever else is relevant to the ATP check in your system then this should not be a problem.
It should be part of UAT though.