I've just done exactly that - loved my H1, resisted an upgrade to H5 and was sort of expecting I might upgrade to the H9, but went on a course where the instructors reckoned I'd "grown out of" my H1 and recommended the 400D and a Sigma 18-200 (11x zoom) lens.
I took the plunge, passed on my H1 to my partner (so it's still in the family) and am on a new learning curve.
Had a few photo opportunities over the Easter holidays and made the best of them to explore the new camera.
I miss the IS, but am getting better at holding the camera steady. Higher useable ISO is also useful. One thing I picked up on the course was using burst mode (3 per second) at slower shutter speeds - a couple of shots will be sharp as you're just holding the button down not pressing it for one shot.
Out-of-the-camera photos are a bit stunning compared to the H1 which was great. And they improve with gentle post-processing. Some of my favourite H1 shots were with the wideangle adaptor - with the 400D and this Sigma lens the wideangle shots are just there without any additional kit and I've always loved the look of wideangle photos. The bokeh with this lens (blurring of out-of-focus areas) might not be quite as pleasing as the H1 but I'm happy with it so far.
I've got some photos with these cameras at
http://www.flickr.com/photos/philllister/
Bear in mind that they're compressed to get on the flickr website and I can't report on print quality as I haven't done that yet. But I'm not disappointed so far.
All in all, I'd say it's worth thinking about taking the plunge. I'm sure the H9 will be fantastic, But with the 400D you've got a bigger sensor so less noise, RAW if you want it, higher useable ISOs instead of IS. I miss the spot metering,but I'm learning how to compensate for that.
All the best
Phill