Since cue Essentials and cue Premium are the same platform, so the short answer is “nothing”.
The longer answer is, your data is your data, and it is there from the moment you create your free account. A subscription to cue Premium unlocks a next level of sophisticated analytics and business intelligence tools, allowing you to slice and dice your data, and to make the best decisions for your business.
Because cue Essentials and cue Premium are different levels of the same platform, you will be able to these more advanced tools to look at your historic data going back to the creation of your account. Conversely, should the business need arise for you to downgrade from cue Premium to cue Essentials, the platform will continue to track everything you are doing, and you can resume using the advanced tools of cue Premium by reactivating your subscription.