How to remove non-UTF values from a table?

An easy question for some I hope.

I have a DB from 8.2 days that when I now dump and try to take into
the 8.3.7, it gives me errors about utf-8 stuff.

I tried searching this list's archives but could not come up with an answer.

Google returns some sites like these:
http://sniptools.com/databases/finding-non-utf8-values-in-postgresql -
but I'm not clear on how to use them.

Following the SQL on this site I could identify some columns that
contain text like this:

"Évolution générale de la situation démographique"

So my guess is that the non-English characters were originally not
getting written in proper utf-8 variants.

Is there any SQL possibility to find these columns and replace them
with utf-8 equivalents using some postgresql commands? Couldn't find
anything in the "Strings functions" (chapter 9 of manual).

We're on CentOS.

Thanks!