Have you found a way to upgrade an Elgg 0.9 installation to Elgg 1.2?

I know it is a common question, but I haven`t found a way to do that for the moment. It was a goal for Elgg 1.5, but they removed it. Anyway, I think some sites have been upgraded yet. Is there any documentation on this? We have build a big community on Elgg 0.9 and we cannot start from zero in an Elgg 1.2 installation. Can you help us?