I managed to unlock and alter the POP Source DOP to birth particles based on a source point attribute. Super.

But saving the node has (predictably) meant that the old unaltered POP Source node has gone, so even using the shelf tools sets things up using my new node, not the factory one.

Not a problem - it's easy enough to grab a copy of the factory node and stuff it back into my Houdini folder, and for now I've just added a switch into my franken-POP-Source node so it defaults to the original behaviour.

But what's the correct process to make a new HDA based on a factory one? Renaming HDAs seems to be difficult after creation, so I'm not sure how I'm intended to… well, “Save As…” my altered node.

use the Asset Manager and do a Copy of the hda - save it somewhere outside $HFS and give it a unique name/label