Now the rna path to nodes happens via the node name, which is ensured to be unique via RNA. As part of this, the node->username string has been removed, upon renaming the node itself it takes care of making sure it's unique (like bones, constraints, etc). There's currently no interactive rename tool, but you can do it via the datablocks editor. - plus a few notifier tweaks, using the newer NC_NODE notifier to refresh graph editor etc.