I did this a little while ago so hoping it's still fresh.
- Insert your SD card to your old phone.
- Go to WA settings and there is an option to back up your chats. Sorry but don't remember which section it's under. Back up to your SD card
- Transfer SD card to your new phone and then download and install WA. DO NOT open the app or go through the setup process yet!
- Download a file manager from Google play store. I'm using Asus File Manager
- Cut and paste/Move the WA files from your SD card to the WA folders on your phones internal memory using the file manager. Copy all the files from your SD card to the correct folders on your phones internal memory.
- Once you open WA you should see an option to restore chats. This will appear, IIRC, after you enter your phone number.
Note: If you already have WA running on your D50 it will not work (uninstall and reinstall WA) and start from the file manager step above