Asset Manager Dialog
Available on the Media menu, the Reassign Media dialog allows you to change the source of media in your document:
How to Reassign media
Click on the media you wish to assign ot another source.
Either select the media from the popup, or press the Browse button and locate the replacement media on disk.
Press Apply Button
Reassignment is not done until you press the Apply button.
Desktop Presenter IP Address
When you select a Desktop Presenter source from the Media list, you will also be able to change the IP address to the Desktop Presenter.
This means you can build a Wirecast presentation with place-holders and when your presenter shows up on site, you can just reconfigure the document to point at the correct Desktop Presenter.