Covert database from ASCII to UTF-8

It's come time to bite the bullet and convert a half-terraybyte database from ASCII to UTF8.  Have gone through a bit
ofeffort to track down the unclean ascii text and repair it but would like to avoid the outage of a many-many hour
dump-restore.

Using Postgres 8.4.X.

Are there any other magic options open to me?
Any way to do an in-place conversion?
I assume slony replication is an option.....
What about some sort of wal log shipping replication?

Any thoughts would be appreciated.