1. Prefills only affect new data
2. If a prefill was provided in the url (Prefill object), this has the highest precedence
3. If a prefill in the form of a Default Value was given in the field configuration, this will be used

Future Feature (not 2sxc 10)

1. The prefill can be different in each language. ATM this isn't possible, since the default language will need to exist, and then translating in another language would copy that value. So it's not clear how this would be done in the UI.