How do I change my default "from" email address for the iOS Mail app?
If you are creating a new message, then the "default" account will be used.  If you are replying to a message in another account, that account will be used as the "from:" address.

It sounds to me like you want to reply to a message received on (say) Hotmail and have the message come "from" a new GMail address. This is not possible to do by default; you will need to change the "from" address manually.