I can't remember if I wrote this or borrowed it, but there are some good lines to work with in this letter. Tailor it to your situation. Hope it helps.
"I am writing to inform you that I have begun a new career in real estate. I have had a burning desire to go into business for myself for some time. The entrepreneurial embers have been burning inside and it is high time for me to fan the flames. Having thought long and hard about this and after much study and preparation I finally earned my license and have joined XYZ Realty, Inc.
Should you have a need for my services or just want to pick my brain, I would be delighted to assist you in any way possible. I want you to know, however, that I am not writing to beg you to work with me the next time you buy or sell property. I simply want you to be aware of what I am doing now in the event that you do want my help or if you have opportunity to refer some business my way.
I want you to know that I am entering this business with a strong plan. I have a vision for how I will set myself apart from other real estate agents. This "announcement" is just one of the early steps I am taking to "get the word out" about my new business.
Any input or constructive advice you may have for me would be greatly appreciated. I hope to hear from you soon. Oh, and rest assured, while I am passionate about my new career I will not be one of those people who hits you up for business every time I see you. Your friendship means far too much to me to risk losing it by being obnoxious."