I just want to say that I don't think you're unprofessional for doing this. I had a post last week where we all discussed letting a family go due to their unwillingness to provide information and we decided it was best to just go with the flow in order to keep good vibes and not "burn bridges". However, I think that in this circumstance, this is what's best for your business. Yes, it is always nice to be able to keep your word and continue the way it's always been, but sometimes it just doesn't work out that way. Sometimes is just doesn't WORK! If this is what is best for your business and your finances (which a HUGE factor in running your own business) then this is what you need to do.

Also, I know how difficult it us to keep "professional and authoratative" in your letters, especially when you've built a more personal relationship with your client. But try to be straight to the point with your letter, and if she has any issues, or comes to you, then you can be more understanding and emotional and more explaination. But for the purpose of the business, and for being treated like a business owner, you should keep your letter straight forward and to the point