When a customer needs more space or more websites, or wants to pay less, switch their package. Their websites, email and files stay exactly where they are. Only their limits change.
Check that what they're using fits in the smaller package. Limits are strict: if a customer is already at or over a limit, they can't add anything more of that thing, whether it's a website, a mailbox or a database. The bars on their package card show what they're using now.
A green message confirms the change, for example "'Pro' changed to 'Starter'". The package card now shows the new name and limits. The Subscription ID stays the same, and their websites are still on it.
The panel doesn't bill anyone. If you invoice your customers through your own billing system, update their plan there too.