The first significant change is to the chat bubbles. Previously, consecutive texts appeared as one big bundle. However, with the update, each message will have its own oval bubble, making it easier to track individual texts.
Old versus new UI for chat bubbles. | Images credit — Android Authority
Another change is to the way that profile pictures appear within the app. Soon, you’ll be able to tap on a person’s profile picture to see a larger version. It should be noted, though, that this particular feature is not available for group profile pictures.
Profile pictures will be able to be clicked on and enlarged. | Images credit — Android Authority
Google Messages will also display the name of the SIM card used to send messages in a group chat. This is particularly helpful for dual-SIM users who want to distinguish between the phone numbers they use to chat.
Google Messages will now let you know which SIM you are using if you have multiple SIMs. | Image credit — Android Authority” 
Finally, the last change that was spotted is to the GIF selection screen, which will soon display categorized GIFs instead of hashtags. All of these changes are expected to be included in a future update to the Google Messages app.
Google Messages will now display categories for GIFs. | Images credit — Android Authority