Linus, I presume you are using the Scripted Validator that looks like this in your Transition:
Screen Shot 2015-08-12 at 17.34.02.png
Please note that the fields available to you will ONLY reflect the fields available in the transition screen so you will need to ensure that the DueDate field is included in the transition screen in order to require the Due Date change. So you will likely find that the transition screen you are using only has the 4 fields you mentioned as options. Please try updating your screen to include Due Date and you should find it appears as an option in the validator.
I hope that helps.
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