Hi,
To achive this you need to enable the Joomla Integration. You'll find it in Acymailing->Configuration->Subscription and it's documented here: https://docs.acymailing.com/setup/configuration/subscription#user-integration
Configure the integration to automaticly subscribe your users to your List B. Once enabled each new Joomla user will automaticly be added to your Acymailing and subscribed to List B.
Next, you want to make sure all your existing Joomla users are present in Acymailing and subscribed to List B. Go to Acymailing->Subscribers and click on the import button. Choose the option to import your Joomla users and subscribe them to your List B. You need to do this only once.
To remove your Joomla users from List A you have to run a Mass Action and setup an Automation:
First you want to make sure that all Joomla users you've imported and subscribed to List B will be removed from List A. This is a one time acvtion. Mass Actions are part of the Automations.
After you have imported your Joomla users and subscribed them to List B you need to remove them from List A. This can be achieved with a Mass Action, which is part of the Automations. You only need to do this once.
Mass Action:
Action:: Remove users from List A
Action Targets: Users subscribed to List B
Now you want to make sure new Joomla users will be removed from List A. This can be done automaticly with an Automation.
Trigger: Trigger based on user actions -> When the user subscribes
Conditions: Execute conditions on the subscriber triggering the automation
Set a condition on lists -> Subscribed to List B and set a relative time in the past
Actions: Remove user from List A
Action Targets: Execute conditions on the subscriber triggering the automation