When updating signatures in Act-On, the changes will not be reflected in any scheduled message sends or automated program messages. You will need to navigate to the automated program or scheduled message, edit the affected message, and save the edit. Note that no actual changes need to be made. The simple task of editing and saving updates the signature to be saved into the message.
Currently, the message preview and email composer will display the changes to the signature; however, they will not actually be placed into your messages unless you edit and save the message.