I've been lazy and haven't upgraded my Magento from December'09. I'm wondering if there's any hazards of upgrading to the current version. I guess more specifically, are upgraded versions isolated (so you need to do them step by step, upgrade by upgrade) or cumulative (no matter what version you upgrade from, the new version will recognize and "fill-in" from your last upgrade)? I don't want to risk any site downtime if I have to reinstall the darn thing...