Not an easy one. Did you have objectguid or ms-ds-consistencyguid as sourceAnchor for Azure AD Connect?
You could try to do a dump of all accounts from Azure AD to a csv file with all the attribute values you need (might have to get them from different sources, AzureAD/Exchange Online) and then use that data to create all accounts again in a new on-prem AD forest. Once all that is setup, add that new forest in your running AAD Connect server and it should match the accounts based on the sourceAnchor.
It will by no means be a quick task but I think it might do the trick.