Congrats on your new role! Hope you're settling in nicely.
Thank you for your partnership in the last years; I'll definitely miss working together. But I'm so excited for your new chapter and hope that we can partner again in the near future at [the new business unit]
Can you let me know who will backfill you so I can get them up to speed on this project?
Congrats again and please don't hesitate if there's anything I can assist.