$ find . -name "*txt" -exec ls {} \;
./foo.txt
$ find . -name "*txt" -exec ls {} +
./foo.txt

$ find . -name "*txt" -exec mv {} aa.txt \;
$ ls
aa.txt
$ find . -name "*txt" -exec mv {} bb.txt +
find: `-exec' に引数がありません

最後どこがダメなの?