Upsizing a prior render

Has anyone asked, or are their any plans to be able to open an existing image and
change the render output options to a larger image?

I'm wondering: What speaks against upsizing your source image to the desired size in PS and then running FF on it?

Yes I did actually manage a work around for this. My task was trying to take a previously completed texture sized at 600 x 600 and to increase it to like 1200x.

I had previously been using PS 5.5 but I now have CS 2, yet my preconception was that
increasing the size of the image would damage it noticeably.

But CS 2 actually did a very nice job of enlarging it under bicubic sampling at which
point I ran it through FF to make minor changes.

But since FF does work so well as a stand alone product that would be a great feature
to have.

Especially since it seems to have an excellent rendering process.

Upscale in 10% increments until desired size is reached. This might produce better results than upscaling in one step. Otherwise try photozoom