Search and replace in all files:
find . -type f -exec sed -i 's/icon-/fa fa-/' {} +
Search and replace in all files: (except .svn directories)
find . -path '*/.svn' -prune -o -type f -exec sed -i 's/icon-/fa fa-/g' {} +
Search and replace in a database column:
Replace 'table' and 'column' with your data.
UPDATE `table` SET `column` = REPLACE(`column`,"icon-","fa fa-");