Migrating from Postgres 8.4 to 9.4 in different data centers
You can set an encoding that is different from the cluster-wide one the following way:

CREATE DATABASE important WITH ENCODING 'LATIN1' TEMPLATE template0;


but the documentation further says: 


  Notice that the above commands specify copying the template0 database. When copying any other database, the encoding and locale settings cannot be changed from those of the source database, because that might result in corrupt data.


All this can be done when creating the database, there is no ALTER DATABASE syntax for changing it later.

Setting client_encoding on the server might be tempting, but that you cannot do for a single DB only, it's all or none.  A slightly ugly idea could be to set this parameter to 'UTF8' upon connection, before any data is accessed.