Not easy, but doable:
Exporting the database:
$ mysqldump -uuser -p --opt --default-character-set=utf8 mabase > mabase.sql
Charset conversion:
$ iconv -f iso-8859-1 -t utf-8 mabase.sql > mabase_utf8.sql
Reimport:
$ mysql -uuser -p mabase < mabase_utf8.sql