I think there may be a relatively pain-free way of achieving what @Herve5 wants.
By default, I believe contacts will be imported to the
Device address book (i.e. the one that does not get synched). Once an /e/ account is configured on the phone, the COntacts app ‘Import from .vcf file’ dialog offers a choice of address books, including
Device and the
Recently contacted address books in the /e/ account.
So I’m guessing that @Herve5’s 500 contacts will have been imported to the
Device address book. From there they can be exported in the phone Contacts app to a .vcf file - presumably v2.1 as that’s what the contacts app understands - then reimported to the /e/ Contacts address book, also using the phone contacts app.
Once the reimported contacts have all been synched to the e.cloud account (check using the web app), the 500 device contacts can be deleted by clearing the data from the Contacts app , as described on this web page.
(Note: Clearing Contact App’s Data will also delete your recent call history and favourites.)
The next time you sync with your /e/ cloud account, the contacts will be synched back to your phone, and will appear in the contacts app.
If you are nervous abut clearing the Contacts app’s data, you can instead choose in the Contacts app (in the three lines menu) to only display contacts from the /e/ address book. The 500 device cntacts will still be stored on the phone, but you wont see them in the Contacts app (unless you choose to do so).
I hope that helps. Good luck!