Here is the main question: What is the new teacher like? Do you know her to be a bad teacher or a yeller? It is quite possible that the teachers discussed it, and thought that the other teacher was better for your son, and they both knew of your request. Frankly, it is unusual that a school would take written requests for teacher placement because most simpley forbid it because it is such a hard process even without all the requests. But, technically, you did make this request in writing, as you sent this to Mrs. Smith.
But, as I said, the real question is, is his placement bad the way it is? Is the new teacher a yeller? If not, let this go for sure. Maybe your son can have a great teacher and continue his relationship with Mrs. Smith too.
If she is not a great teacher, know that they cannot all be gems, but pick your battles carefully. Not only would you be making yourself a reputation with the principal, but you will also put Mrs. Smith in a bad spot.
They will not all be gems. A very few will be bad. A very few will be outstanding. The vast majoirty will be good teachers with a combination of bothe bad and outstanding charachteristics based on what you child needs at the time. The funny thing is, teachers all three categories switch around for every student. I have been shocked that someone raved about some teachers my children have had that I thought had no redeming educational qualities, but for thier child, that was an outstanding teacher.
One big caution about email. Write HOT. Write everything you are thinking, say what you want to say, get it all out, but the one thing you should never write when you are mad is the email address. Save that unaddressed email for a few days and look at it again. If you are still mad, let it sit a while more. Write hot, but send cold. 99% of the time, you will find yourself toning it down drasticly, or deleting it completly and letting it go. This will probably be one of the times you use theraputic writing and delete the email and let it go.
M.
PS: administrators really don't like to hear how much you volunteered when you are discussing an educational request. Parents don't earn educational advantage by volunteering, and you can bet, if he ever granted a child a benefit because a parent volunteered more for the school than others, he would be behaving in an unethical manner. There are people who scrutinize who gets what, and if anything, this is why they do thing fairly based on children's needs, and usually refuse to even consider placement requests.