>>I've had a similar experience inthe past. What I did was every time there was a bug reported in a screen I took that opportunity to rewrite that screen. It spread out the cost of a rewrite and over time the code got a lot more stable. Well worth it in my opinion.
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>What about database design? This is not easy to change / practically impossible.
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>Also in this case most of the screens are dependent on each other.

In my case the screens where pretty independent of each other. I left the database denormalized but did go and remove duplicate index. That was a quick search to find where they where using a duplicate index and replace it with the first index. It wasn't ideal but because of the cost I felt it was the best trade off.