samba - May 2004 - Migration: 2.2.3 to 3.0.4 on solaris 8 / german umlauts

we have 2 machines running with solaris 8 samba.
2.2.3 on the 1st and samba 3.0.4 on the 2nd.
The 2nd is backup, shares are the same. Switching the machines is done
transparent for the users.
locale is iso_8859_1 on both.
I tried to use "unix charset = ISO8859-1" and "unix charset =
CP850" in smb.conf, but it didn't work.
Using convmv ist no good idea since ~2TByte would have to be converted (lots of
small files).

Is there any way to have correct "umlauts" in both samba versions?

Thanks
Oliver