An open degree sounds like a great idea. And I guess it comes down to either choosing courses that have a direct relevance to your chosen career or a course that you will enjoy and gives you some transferable skills as well or a mixture of both.
I think the Understanding Systems and Managing Complexity combination would be OK as I can see relevance in both.
In terms of the other courses;
Understanding Social Change - I agree that it looks specialised, but what I guess you would get out of it is how people react to change - which would be applicable whether looking at introducing changes in a bank or in a school or in a hospital or anywhere really.
Networked living - Very IT focused, but so many Change situations will involve an element of IT that it could be useful background info, if you don't have this already.
Ethics - I guess we all have our own ethical position anyway. This looks very theoretical.
Engineering - there is some specialised stuff in there (fundamental laws of physics - eek!)but the majority of it seems like a good grounding in design and engineering concepts.
Innovation - looks like a good bet too - any change will require an element of design and understanding the design process is a good place to start.
Working in the environment - very interesting, think this may have limited direct application in the world of Change Management (I might be wrong and will depend) but looks like one of those that will give general skills and be interesting.
Managing in the workplace - looks like a good one, lots of transferable knowldge.
I never knew that the OU did such interesting courses! I don't think you'd go far wrong with any of them - which probably doesn't help you choose, but hope that gives a wee bit of insight.