I just wanted to let you know that the change worked, and the new conversion no longer has images overlapping text. The images are quite small, but I expect that's due to the width of the image in the original epub file, and that only the height was corrected.
On another note, thanX to the response to dwanthny above, I've discovered that by using the tweak epub function, I can replace the cover.jpg file directly in the epub without converting it. However, if the file did not already include a titlepage.xhtml, I don't think the cover.jpg file will be shown by the reader. I'll have to play with this some more to be sure.
So if I'm right, changing the cover in the metadata in calibre does not change the epub file, and is therefore only a local change.