Hi Ken - When you ask - 'Is there a way to set up or modify a collaboration sync so that a new item on the guest portal updates a pre-existing item on the host portal? " - The short answer is yes, but it depends on the type of collaboration you want to use and that's based on your data update workflows.
For us, editors update SDE layers, those layers are extracted to a file gdb each week. The file gdb layers are shared to our enterprise portal as hosted feature layers from the arcpro sharing module / api.
Each week python scripts overwrite the service definition files, updating the hosted feature layers in portal. The layers are then shared to groups that make up the guest portal workspaces for by-reference collaborations, where our guest portal sends the layers to our agol host.
On our agol host, the groups (workspaces) that receive the content from portal are shared with Open Data, and in this way we update data in Portal, it is seen in Open Data.
In fact, once the layers are in place, the by-reference collaborations don't even need to sync in order to see data updates to the layers in the host org because they are just pointing back to our Portals' hosted server site.
There is an 'as-copies' method too that works best with send-receive types of workflows to keep the data updated. In this case, you would share a view of the agol layer with an Open Data group....
Hope this helps-
David
