Yes. It takes some effort to do it, though. Personally, I use Gary's "Lejendary Levels" idea (one which was both liked and reviled) to make it easier.
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Serl's Corner
It's possible, but in the end I found it to be too much trouble--there are so many thing different about the LA creatures, that you can't easily substitute them. And it is harder to convert NPCs. But it can be done.
