When creating a new translation of an object, it won't be possible to change the base translation if you don't have all the required fields.
1. Create a new Article in English and publish it
2. Create a new translation in, lets's say, Finnish but choose no base translation
3. Go to the left panel, and select the existing the English translation.
4. Click the view button
5. Confirm you will receive and error indicating the summary is required
6. Insert any text into the summary
7. Go to the left panel, and select the existing the English translation.
8. Confirm the translation will change