converting some rows from utf-8 to iso-8859-1

if a table column is supposed to contain text in iso-8859-1, but utf-8 encoding have snuck in on a few rows by mistake, how are these rows converted into iso-8859-1?

what i am looking for is something like this:

update mytable set mycolumn = utf8toiso88591(mycolumn) where id between 500 and 600;

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