In a MIF file, you could try a global search/replace for <FrameType Below> to <FrameType Inline> within the context of the <AFrames ... > section.
And Rick Quatro has a Framescript that does this.
Thanks for the ideas. Concerning Rick Quatro's script I found several references leading to deleted Adobe pages, but nothing on his website. In case this is free to use, do you happen to have a (still working) link?