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 Orders→ Manifests.
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.