Can I upgrade to SQL*Net V2 if I still have V1 clients?

SQL*Net V1 cannot talk with SQL*Net V2, and vice versa. The only way to overcome this problem is to run SQL*Net V1 and V2 simultaneously on the same database server. You can then install SQL*Net V2 on your clients as time permits. SQL*Net V1 and V2 can coexist on the same server, or on the same client.

You can also list V1 connect strings in your TNSNAMES.ORA file. Eg: ORA1_NET1 = T:machine_name/port:database_name

Note that SQL*Net V1 is not available from version 7.3 of the database.