I've done the flatten everything down method and I've done the working off of my existing map thing (after resetting my previous island, where I'd flattened and rebuilt) and boy I had way more fun with the latter. I did end up changing things pretty drastically (although not 100% -- I still have most of my substantial cliffs in the back and there are portions of my river that match up with the original), but it was just way easier to build off of what was already there and work in chunks across the island than to handle a completely blank map. I liked what came out of it much, much more than with a flattened island, too.