@JefflyJ6  - You can leave the Assignment 2 as it is but remove the incoming connectors to it (leave the outgoing connector to Assignment 3 as it is)

So, connect Assignment 1 to Assignment 3.

This way existing tasks in Assignment 2 can be worked on and once submitted they will move to Assignment 3.

New cases will go to Assignment 1 and then to Assignment 3.

If Assignment 2 is no longer needed in the work flow for any scenarios in the future, then the recommendation provided will work without impacting the inflight cases.

But, if there is a scenario that needs to consider Assignment 2 in future, then I would recommend circumstancing the workflow. 

​​​​​​​@JefflyJ6 3 ways we can handle 

1. As mentioned connect Assignment 1 to Assignment 3 - Drawback what happens to the current case in that assignment , we have to think on this direction as well.
2. Circumstancing the  flow - Works if application is small - Drawback difficult to maintain - to be the last option.
3. For immediate solution we can have the Decision shape introduced between the Assignment 1 and Assignment 2 , such that all the new case would by pass Assignment 2 and new case would route from Assignment 1 and Assignment 3 - Here your current case on Assignment 2 is also taken care and New cases as well.

Hope this helps, Suggested the above solutions based on some common scenario we face.

Thanks

Chandan U