What is the best way to manage change release for existing process model outside

There are some best practices that we follow within Appian for change management. It starts with packaging the application into logical unites (such as Baseline contents, Environmental specific contents, CDT's. Data stores, and Application specific contents). Then when it comes to migrating to one environment to another, you can either do a full migration or a delta. I personally prefer delta as it's easier to manage but you need to have a solid build structure and practice in place for all developers to follow.

In your case, what you've done seems to be a delta release (correct), what you do want to do is use your ticket management system to maintain the release notes. I feel it'd be a good thing to have the spring releases recorded along with the sprint cycle, UC definition rather than within Appian. If you do it within Appian, (you can, by all means), then you need to find out how this will be used by the end user. For e.g. if you package it as a document within the application, then the document won't be visible unless the package is deployed - which defeats the purpose. So both Ticket tracking and release management should go hand in hand. As a matter of fact, you should also have version control in the mix that holds the version of the artifacts (or the export) that you migrated across.