Yes what you've seen is correct.
Think of it as a space. HD is capable of having 1920 X 1080p (example full boxes) space and you're trying to put 420i (half of boxes) to 1080p. So you have to position your boxes properly so it may look like the 1080p space is full.
That's why viewing High Definition with Standard Definition source look undesirable.
While viewing on Standard Definition from High Definition source would be better, but TV with SD will not no run on HD source. I haven't check on why it will not work so you might ask somebody else on that.
That's why they're selling upconvertion/upscaling unit these days. It takes your SD 480 resolution and fill up the rest of the space of up to 1080 HD resolution.
Now you might be wondering about what units are........?. Here's one link and you can go on from there.
http/www.hkflix.com/xq/asp/infoID.35/qx/info.htm
unfortunately there's no other way around this since that is where the technology is taking us.
Hope this help you somehow.