on latest version is not possible, the only possibility for this is downgrade your jomsocial.. You can get the old package from here
www.jomsocial.com/download-old
1) If i downgrade, then every time i have an issue you will say "upgrade to latest version". Or not?
2) Is there a way to have the new chat feature enabled and also have the old messages in "inbox" and "sent", as chat old messages?
Now there is only the message "This is your inbox Conversations will start showing here once you have your first chat with someone. You can start a conversation with your friends any time. Start a conversation"