It is indeed possible to reassign a courier once manifest has been generated. Described below is the process which helps you do so:

1. Go to OrdersManifests.

2. Here click on the Manifest ID as shown in the image below:

3. It will open up a new screen which will have your Order ID. Click on your Order ID.

4. Scroll down below to find the re-assign button. Click on it to choose a different courier partner for your order.