GIMP :: Remove Transparency In Animated GIF?

I have an animated gif in which I put in a transparent background on all the frames a long time ago. But the edges of the images were all anti-aliased, so the edges look pretty bad on a much different background. Now I want to change it back to the black background on each frame, and the origial file is lost.

I went through each frame and used Select by color to select the transparency, then Edit to fill the selection with black. However, most frames have a layer boundary that is smaller than the image size because the animation does not always fill the screen. The background on each layer/frame is still transparent. When I saved that work, only the current layer boundary in each frame shows - the rest is transparent. So instead of an unchanging size image with a black background, I get a variable sized one.

So I thought I would remove the alpha channel from each frame. That looks ok while the whole image is open in GIMP, but when I go to save it, GIMP puts the alpha channels back in each frame! I don't know why it's doing that. But I did notice that when I preview the animation with the alpha channels removed, artifacts are visible when the next frame is smaller than the one before it, so I don't know that it's what I want, anyway.

Can I change the alpha channel from completely transparent to completely black? how to make the blackgrounds transparent, but I can't find anything about undoing it.