Scenario 1: This is expected behavior.  Internally in K2, each process version is its own assembly.  When an instance of a process is started, it uses the assigned assembly throughout the lifetime of that process instance.  When a new version is deployed, new instances use the new version of the process but existing instances continue under the older version.

Scenario 2: There is a process version migration utility available from the community.  However, it has limitations and does not work in all instances.  Essentially the utility's ability to migrate process instances boils down to the types of changes that are made between versions.  The documentation that comes with the utility offers more details.

With regard to Scenario 2, you can use K2 Process Management in Visual Studio to retry a process using a later version. Provided you have installed the blackpearl client tools on a machine with VS, you should see a "K2 Process Management" menu item under the "View" menu. Select the correct environment and it should show all processes in error under the Errors node. You can then check the process to retry, click the Retry button and select the process version to retry at. You may need to use the "Goto Activity" function in the workspace to start the affected task again.