Nobody (not even WhatsApp) will be able to access your messages. If you lose your phone, WhatsApp doesn’t store your messages on their servers.
Your calls, photos and videos are free from prying eyes. This means that while no-one can see your messages, you should still be aware that malicious third party apps that can record audio and screen shots can lift your data from your phone.
WhatsApp has enabled a security setting which allows you to receive notifications when a contact’s security code has changed, but it’s turned off by default.
You can still move your account to a new phone or new number, but for a number, you have to ensure that it’s changed on your old phone.
Because of the new policy, backups of your messages will be directed to your Google Drive account over Wi-Fi by default. You can also manually backup your data.
You can still send chats via email and but be aware that the emailed chat is saved in plain text, making it vulnerable to criminals.
Photos and video
Despite the encryption, WhatsApp images and video will still show up in your phone’s galleries. You can hide them on some phones by using the show/hide albums button. You can also use a File Explorer and create a .nomedia file in the WhatsApp images folder on your phone to hide them.