Short answer
HubSpot's KB: messages to your WhatsApp number create a new thread and ticket in help desk. If the account is already connected to the conversations inbox, delete that channel first. Account Access permissions are required.
1. In HubSpot
The relevant screen in my demo portal.

2. Worth knowing
Plan the switch so no messages land in a deleted channel.
3. Related
WhatsApp in HubSpot and subscription statuses.