Manual:$wgLegacyEncoding

Language, regional and character encoding settings: $wgLegacyEncoding
Set this to the character set, which you used before you updated to MediaWiki 1.5 (eg 'ISO-8859-1'). MediaWiki will then perform a temporary character set conversion when loading old revisions, which are not marked with "utf-8" flag (revisions created with MediaWiki 1.4 and earlier).
Introduced in version:	1.4.0
Removed in version:	Still in use
Allowed values:	Unspecified
Default value:	false
Other settings: Alphabetical | By function

Details

Set this to the character set, which you used before you updated to MediaWiki 1.5 (eg 'ISO-8859-1'). MediaWiki will then perform a temporary character set conversion when loading old revisions, which are not marked with "utf-8" flag (that are revisions created with MediaWiki 1.4 and earlier). You can use this when you convert a wiki to UTF-8 and you do not want to do a mass conversion of old text data. Note however, that old texts, that have been converted because $wgLegacyEncoding was set, are not updated in the database! The next time they are needed, they will have to be converted again.

This DOES NOT touch any fields other than old_text and ar_text (and also does not change their content). Titles, comments, user names, etc still must be converted en masse in the database before continuing as a UTF-8 wiki.