Replace part of a string in MYSQL, when only the substring's starting characters, ending characters & a unique string in the middle is known?
Test:

UPDATE source_table
SET field = CONCAT( SUBSTRING_INDEX(field, @start, 1),
                    @replacement,
                    SUBSTRING_INDEX(field, @end, -1)
                  )
WHERE LOCATE(@middle, SUBSTRING_INDEX(SUBSTRING_INDEX(field, @start, -1), @end, 1))