Change encoding of SQL_ASCII database with multibyte chars?

Hello, there! I recently migrated a medium-sized db from SQL Server to
postgres. A couple months after the migration, I noticed I was having
some problems retrieving some data via the JDBC driver. The problem
turned out to be multi-byte characters (smart quotes, etc) in the
SQL_ASCII database.

As I understand it, I need to change the encoding to UNICODE in order
to support non-English characters, smart quotes, etc. The trick is that
I would like to preserve the small quantity of multi-byte characters
that are already in the database.

I've tried dumping the db, changing the encoding in the CREATE DATABASE
command to UNICODE, and reloading the dump, but I get errors like
"ERROR: invalid byte sequence for encoding "UNICODE": 0xfd3130 ",
which makes me afraid that I'm losing data.

Is there any way to re-encode my database that will turn the (possibly
mangled) multi-byte characters back into their original form?

Thanks for reading....

Leon