Thanks for dropping by here today, @SaraS2. I'll answer your concerns to ensure your client retains important features when downgrading subscriptions.
Please be aware that once they've downgraded their subscription from QBO Advanced to Essentials they will lose the markup and billable expense because this feature is only available in QBO Advanced.
In addition, here's an article if you wish to continue to downgrade your current subscription: Upgrade or downgrade your QuickBooks Online subscription.
You can also check this page to help you update your QBO subscription information: Manage billing, payment, and subscription info in QuickBooks Online.
It's been a pleasure to have you here, @SaraS2. In case there's anything else you'd like me to help you with or need assistance with your QuickBooks subscriptions. Feel free to click the Reply button below. I'll be more than happy to help you out again. Stay safe!