An Engagement that has already been commenced for a given client (or company) can be modified at any time.

To modify an Engagement, click the Engagement icon for a given client as found in your Clients listing (the handshake icon, found towards the lower right):

Doing this brings up a display of their current Engagement, like so:

From there, click the "Modify..." button found at the bottom of that pop-up window.

This brings up the Engagement editor pop-up.  

From there, you are free to modify anything about the Engagement, including:

• Increasing or decreasing the total allocation
• Changing the rules that dictate which appointments do and do not count
• Setting or resetting notifications
• invoicing plan details, including the date of the next automated invoice