If you buy Software licences, they will live alongside the Core ones as well as give Core to the Software users (so you could have 25 Core users and 10 Software users for a total of 35. The Software users would have Core access too).
If you need the more simple case, such as you have 25 Core licences and simply want 25 Software instead, then yes, contact the licence team via my.atlassian.com and you'll be able to simply pay the difference.
I have multiple projects that use variations of the same base workflow. The variations depend on the requirements of the project or issue type. The variations mostly come in the form of new statuses ...
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