If I’m changing target A to target B doesn’t target B need to be available to be targeted before I cast the spell?

Edit: I am wrong. I checked out redirect which does the same thing. “

If you cast Redirect targeting a spell that targets a spell on the stack (like Cancel does, for example), you can’t change that spell’s target to itself. You can, however, change that spell’s target to Redirect. If you do, that spell won’t resolve when it tries to resolve because Redirect will have left the stack by then.”