Considering taking a step down to move departments…

Any advice on negotiating salary and generally dealing with feeling like your moving downwards rather than up the ladder? Also a little nervous as I think my manager could be younger than me with less years of experience in the industry, and I am unsure how to approach that without downplaying my expertise or stepping on toes.

Looking to stay in the same education area of the industry, but move from product to marketing. Partly to gain experience beyond the scope of my role to put me in a better position for SLT roles. Partly because I’ve been told by colleagues for years that my skills would really suit marketing and I often work closely with marketing and take on a lot of that responsibility voluntarily because it feeds my creativity. Role would be Product Manager -> Marketing Executive, going back to a company and team I have a very good reputation with.