thanks for the clarification. It's as I said: You can modify the process in the designer and publish the modified process to your process engine. As long as it has the same version (Definitions -> Deployment -> Project Version) it will completely replace the previously deployed version. I know that modifying arcs was a pretty troublesome in the past, but with a recent version of AxonIvy (I guess something 8ish) this should work - if it won't work however, the deployment will fail.
Your already started tasks will simply use the newly deployed version, because the version you deployed does not represent a new PMV (process model version).