How will a career change affect my application?

Hello Enzo218,
Before answering your question it's probably helpful to know how long you have been in your job and in sales/development and to see what some of your accomplishments have been. Admissions committees are going to look at the scope of your work and consider how you have progressed (have you been taking on increasing levels of responsibility, did you receive any promotions, etc). Hopefully you've been in your role long enough to prove that you have been able to make an impact and that you've acquired useful skills. If you feel that now is a good time to transition into a new role, think about how this will be relevant to your post MBA goals of working in marketing. Gaining knowledge around PR could be useful for your future roles and you should check to see what kind of responsibilities you will have in the new role to make sure that you'd be happy with that kind of experience. Also, for someone considering brand management, it will be important to demonstrate that you also have strong analytical skills since a lot of the marketing work that you do will likely be quantitative in nature (managing budgets, forecasting sales, evaluating costs and finding cost-savings, etc).
Overall, you want to make sure that the timing is right for this move and that you have a good reason for making the change (which is sounds like you do). Good luck!
Take care,
Dina