In order to merge databases (msgstore.db.crypt12) you need the key.

Decrypting the message store database

The easiest way to get the key on a rooted device with whatsapp still installed. You can find the key in /data/data/com.whatsapp/files/key.

You can then use an app like omnicrypt (no longer on Google play) to decrypt it. See [1] (don't know if this is obsolete)

OR you can try using adb and the Whatsapp key/db extractor

OR you can try extracting the data from Google Drive, but the Whatsapp Google Drive Extratctor doesn't seem to work any more

If your old device doesn't have the key any more

You can still get a new key by backing up your old databases (/sdcard/WhatsApp/Databases) and then putting the old database you want to look at

Whatsapp viewer (also exports to txt html json and allows you to search messages - originally from [3]