Whatsapp backup for new and old chats
If these messages are important, you can root your phone in order to restore them.
There are many methods to root Android 7 to use at your own risk.
WhatsApp make a backup of the messages database each day at 2:00 AM, these databases are stored encrypted in /sdcard/WhatsApp/Databases/. The key used to encrypt the backup is stored in /data/data/com.whatsapp/files/key which can be accessed only if you have root privileges.
To restore your previous messages:

root your phone
enable USB debugging
dump the encrypted databases

adb shell pull /sdcard/WhatsApp/Databases/ 


dump the key

adb shell pull /data/data/com.whatsapp/files/key


use this python script to decrypt the encrypted databases: