When a study has been completed, you need to:
1) lock your records,
2) export your data,
3) turn your study offline, and
4) remove all user rights that you will not be needing.
For example, you can leave yourself as the only user, and you can remove all your user rights except View, Export, and Manage Records rights. Other users can be removed completely. This means that you will have access to the data in the future.
The data is stored by default for 15 years on our server (in accordance with GCP rules) without additional cost