Dear Shrithar,
Do not enter offset at this stage.
Please follow below rules while data migration.
Create Function Location, equipment & Tasklist with Old Validity / key date (Validity management).
Then at the time of Maintenance scheduling using IP10 transaction, donot enter offset. Rather, enter the Last Maintenance Order completion date & note its Maintenance "package select" button.
Your problem will resolve.
Regards
Santosh