I have worked for a fairly new, small-medium US-UK company for 18 months. My role was newly created in a certain part of the business and I was essentially head hunted and persuaded to join because the leadership at the time recognized that the needs of this part of the business were different and not covered adequately by an existing role.
It was recognized by all that the incumbent in the other department was not happy with this but both leaders sat down and scoped a slide deck and job description documenting my responsibilities vs his and a generic type statement that we would work together in future to navigate those "grey areas".
Fast forward to this year, the incumbent has moved into a central services role and promoted above me. He presented a slide to leadership without consulting with me which essentially removes a huge scope of what I do and reduces my role in many areas from decision maker (per job description) to SME and requires the same "tool kit" to be used even though it is not directly applicable to my space.
To avoid outing the industry, if we were in the baking his job title would be "Ingredients" and mine would be "Selling Pastries". Our job descriptions both include "Ingredients, baking, decorating" for cakes and pastries respectively. My role has more focus on Selling and his on buying kitchen equipment.
The proposal agreed by leadership has him doing Ingredients, baking and decorating for pastries as well as selling cakes. His argument was that "Barbara is still selling pastries" so no impact and as they didn't know the detail (and HR were on holiday) they appreciate the impact beyond the job title. Hope that makes sense 
I have now raised with my manager and HR. HR have been good and want me to document the impact on my role and remit. They indicate they will be looking to see extent of the change. What HR-friendly terms should I use? eg material change?
If it is relevant (and I think it is) my manager and this guy are in the US. I am in the UK - as is our head of HR so there is some learning for the US team regarding UK employment law here. We do have an employee advice line but I have not yet called them.
Thank you in advance.