"Distribution tasks with corresponding series tasks may only be retargeted to a different package."

It is neither a permissions issue nor a one-off issue. This restriction has been in place since 2011. You probably only think you've made the exact same change in the past, because the way multiple tasks interact here is not very intuitive.

The problem is that series tasks on packages (like the Ubuntu/quantal xserver-xorg-video-ati task) are defined in a very strange way: they affect all tasks on the distribution in question. This means that, even though it doesn't seem as though it should be affected by that other task, changing the Fedora xorg task's distribution from Fedora to Ubuntu would require Launchpad to create an additional task for Ubuntu/quantal xorg, which would be a very strange thing to do when you just changed a task's target. Launchpad therefore refuses to make this transition.

Ideally, I'd rather that series tasks weren't defined in this way: I think it would be clearer if you only targeted tasks for a single source package to a series, rather than targeting all the tasks on a distribution at once, and then we wouldn't need to have this strange restriction on target transitions. But that would be a big and complex change.

As it stands, the simplest thing for you to do is just to mark the Fedora xorg task invalid (as you've done) and then use "Also affects distribution/package" to create a new bug task as appropriate.