"When I open a Gmail conversation, the most recent message shows up on the bottom. Can I change this?"
SHORT ANSWER: No :( Sadly, Google won't let you move the newest messages to the top of a thread. But if you hate conversations, these can be turned off completely if you like.
CONVERSATION VIEW: Google does not allow you to change the order of the conversations within conversation view. Once you open a conversation, the most recent email in the thread will appear and you will have to click on the emails above to view older messages in the conversation.
TURN CONVERSATION VIEW OFF: If conversation threads are not your thing, go to the main Settings page, look for the “Conversation View” section, select the option to turn it off, and save changes. If you change your mind, you can always go back.