This Gimp plug-in interlaces/deinterlaces all layers of a film (multilayer image).
interlace: The number of layers is divided by 2 merging two successive layers into even/odd numbered lines of one layer. You can keep the image height by dropping lines or double it and keep all the lines.
deinterlace: Even and odd numbered lines of each layer are stored into two separate layers. Thus the number of layers is doubled. Lines in between are interpolated or skipped (selectable), i.e. the new image has the original height or half of it.
You find it at "/Filters/Animation/Interlace Tool".