I do not understand the question, it actually looks like two unrelated things, and neither is clear. I'll try to explain, to hopefully help you give us a bit more detail:
>Change workflow according to selected field
Workflows are determined by the project and issue type, not fields. Do you mean you would like to determine the issue type by a field, or do you mean you want the workflow to do different things depending on the content of a field?
>Cannot change sub categories according to selection from previos.
What is a "sub category" in this one? And what "selection from previous"? And what exactly are you trying to do with the sub-category
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