Hi @Kalo and welcome to the Community!
If I’m understanding your use case correctly, the original v2 POST /api/v2/list/{list_id}/members endpoint that you mentioned is the Add Members to a List endpoint. An approximate equivalent for the new v3 API would be the Subscribe Profiles endpoint.
The main difference here is that the v2 endpoint is intended to act similarly to a .CSV import, meaning that profiles added with this endpoint are assumed to have already supplied consent. With the new v3 endpoint, you can still add multiple profiles to a list, but it’s now impacted by double opt-in.
This means that even if you’re moving customers from one list to another, if you have double opt-in enabled, those customers moved to the new list via API will receive a confirmation email.
I hope this helps and thank you for using the Community!
- Brian