Oracle Database access

We run a mixed unix windows environment. Windows users can access oracle databases on the unix servers using windows authentification.
We are upgrading our windows domain and creating new user accounts.
Problem the new accounts do not exist in the oracle database. We need the new users to appear to still have the same account credentials as before.
Does anyone know how to map the new accounts so they have an aliase or spoof their previous account name? I think name mapping will only work for unix accounts not the oracle database user name. Would it be possible to use the name mapping to map a winows account to another windows account?