A good welcome letter as their new sponsore would be, that you are there for them no matter what!
That they have your support at any time. I hope you will enjoy to work with me here and please don't hesitate to ask me anything that you want to know about SFI and your progress here.
You need to encouraging your new affiliates, emphasis to them that this is not a demotion. They are self employed people who running their own businesses.