Sync CRM and ERP after ERP-systemcopy

Hi experts,

after a system copy from P-ERP to D-ERP our D-CRM and D-ERP are not synced in terms of Customers

What would be the exact steps to successfully download all Customers from D-ERP to D-CRM again?

Do we need to delete all existing Customers in D-ERP before? If yes, what happens with the old transaction where the old customers numbers are maintained ?

Thanks in advance,

Wladimir