I wouldn't either. An update would consist of information that belongs in your AMCAS if you were to fill it out now. That is, if you had started a new job/volunteer position, or you had additional pubs/abstracts/posters/accomplishments to add, that would be considered an update. Telling the adcom what you're NOT doing won't be helpful to you or them in improving your applicant profile.
I can see your thought process from the intentions of a "letter of interest" standpoint, but again, I don't think telling them what you chose to give up for the potential of starting med school is delivering the right message. Usually you want to talk about why that particular school should interview/accept you in that sort of missive. Highlight your interests and goals and why the school is a good fit for you to pursue them.
Hope that helps. I wrote an update/letter of interest for my dream school recently, and I had a person that's on the adcom look over it. He had two big pieces of advice: not to repeat anything in my AMCAS because they'll have it with them when they read my letter and don't be coercive or demanding in any way. Some phrases can be risky. If it's a reach, they'll perceive it as such. Best of luck.
