You can do this:
#!/bin/bash
find . -name "*A.jpg" | while read f
do
newname=${f/A.jpg/A@2.jpg}
echo convert "$f" -resize 400x400 "$newname"
done
Remove the word echo
if it looks correct, and run only on files you have backed up.
You can also do it in a one-liner, if you really want to:
find . -name "*A.jpg" -exec bash -c 'old="$0";new=${old/A.jpg/A@2.jpg};echo convert "$old" -resize 400x400 "$new"' {} \;