Is it too late to wean my 9 year old cat off her litter tray??

Pretty much, although you could try moving it outside.

You might as well try to move a mountain; but why do you want to do this?

You could try, but a lot of cats don't like changes to their routine or environment. If you do try, prepare for revenge messes, probably on your bed or other places that your scent is very heavy on.

And not to be an echo, why would you want to stop using a litter box? Most litter training questions are on how to start, not on how to stop.

What would you want to replace the litter box with?