Thank you, that’s a great question.
Technically, you can detach a subscription, same way you attach it. simply use ‘sudo pro detach’ command.
That said, it’s possible that you’d get rid of a VM without detaching a subscription. Then, you might struggle to find a way to detach it.
This is why we don’t prevent you from attaching more machines than the number of entitlements you have (either free or paid). Instead, we monitor how many active machines you have at any given moment. In other words, you should ensure that the number of active machines does not go over the limit.