This resignation letter is due to a promotion within the same company.
I hereby tender my resignation as (position) at (Company), effective (date), as I have accepted the promotion to (position) in (department). While it's difficult to leave a position that I have loved and coworkers whom I greatly admire, I am excited to explore the other opportunities available to me at this company.
I am deeply appreciative to you for helping me reach this new place in my career. I have reassigned (projects) to (coworkers) and I am prepared to spend my final time as (position) training my replacement and finishing any additional work required of me.
While I will only be a few doors down, I am going to greatly miss this office and the welcoming, supportive atmosphere created by this team. Thank you for everything you've done for me.