Need Shell script help!

I have written a script that substitues one parameter for another parameter in a file. How can i fix it to still do this but without making a backup file if the file is unchanged.
This is what I have:
#!/bin/csh
set firstargument=$1
set secondargument=$2
set filename=$3

for $1 $3
do
mv $3 $3.bak
sed "s/$1/$2/g" $3.bak > $3
done
Please Help!

one way would be to do a diff of the files, and if they're the same delete the backup