For the UI Unification Upgrade, we will be auto-updating your production account with the "Switch to integrator.io" link. These auto-updates are not able to be pushed to your NetSuite sandbox accounts and your sandbox account is not automatically upgraded with your production environment. The sandbox environment will need to be linked separately.
You are able to manually update your Celigo Integrator Bundle in your NetSuite sandbox environment in order to complete the UI unification upgrade. To upgrade your sandbox environment, navigate to Customization -> SuiteBundler -> Search and Install Bundles -> List -> Celigo Integrator Bundle (with id 20038). From there, you will click "Update". Then, the "Switch to integrator.io" link should be available to be clicked to complete the upgrade process.