Congratulations on your new job as a [indicate title of new position]! We’ve witnessed your diligence in work, and your efforts surely bore fruits. I know you can maintain that great performance! I can imagine your joy in facing a familiar area of work. Know that we will certainly miss your cheerful personality, great laugh, and your helpfulness with any of our problems.
We would be thrilled if you can drop by some time. Don’t be a stranger!