sed command in dry run

Remove the -i and pipe it to less to paginate though the results. Alternatively, you can redirect the whole thing to one large file by removing the -i and appending > dryrun.out

I should note that this script of yours will fail miserably with files that contain spaces in their name or other nefarious characters like newlines or whatnot. A better way to do it would be:

while IFS= read -r -d $'\0' file; do
  sed -i 's/string1/string2/g' "$file"
done < <(find ./ -type f -print0)