I want to move existing instances running on the old version of the workflow into the new instances, can someone please help and guide as to how I can do this. I need to provide feeback to management ASAP as to wheher it is possible or not not and what are the risks associated with it.
I don't think that there is a way to do this.
The risk would be to high, that the "Workflows" run in an error because there are missing information or something like this.
What you can do is that you restart every Workflow manually and say "Go to Activity".
But the risk that the Workflows runs into an error are also there.
You can migrate process instance running in one version to other version using one of the following way.
One of the tool is Process-version-migration-utility (you can get more tools here)
This will be best solution as here you can handle the scenario like
You have introduced new data field (this data filed was not in old version) and this data field is required some valid data to run the process.
This link will help you to know more on this.
Let me know if this helps you.