Use the Up key to traverse from bottom of To, Cc, etc, addressee list. When the top is reached the Up key causes traversal to the From entry box. If the Up key is pressed after the From entry box is reached the Up key changes from causing Entry box traversal to causing scrolling through the list of potential From addressees; and causes a similar change in behaviour for the Down key. It is far, far, too easy to overshoot when using the Up key and cause the change in behaviour without noticing: and consequently change the From addressee and perhaps not notice that either. The behaviour of the Up key should not change in this way. The User Interface is simpler if it does not change in this way. A simpler User Interface is a better User Interface. There is a drop-down list of From addressees right next to the From entry box: why not rely on that rather than mess with the behaviour of the Up key.
The Up key is using the drop-down list. You might find more comfort in using the TAB key and the Shift+TAB key combo.
I never thought to try the TAB key: possibly because tab traversal usually gets trapped by typing-mode on reaching an editable widget and I have learnt to not use it. Removing a few tabs inserted by accident is more acceptable than having to remember what email address was overwritten when relying on Up and Down keys. Thanks.