Hi Ann,
Unmanaged solutions only reference (and don't own or contain) customizations.
You will learn more on solutions here: https://docs.microsoft.com/en-us/powerapps/maker/common-data-service/solutions-overview
Solutions are used to group customizations and be able to deploy them, following a healthy Application Lifecyle Management process (i.e. deploy from a development environment to a test environnement and then to a production environment).
It is a best practice that we strongly recommend to only do unmanaged customizations in your development environment and to deploy them in a managed state.
Why managed? Because then your customizations are somehow "owned" by your managed solution, and deleting a managed solution removes the customizations it contains from your environment.
On the contrary, deleting an unmanaged solution actually does nothing to your customizations: they still exist in your environment. You've just deleted the container that referenced them.
It's possible to create a new unmanaged solution to reference all the customizations that were part of your other unmanaged solution, but you also need to consider the impact it's going to have downstream, on the environments were these solutions had already been deployed.
Now, that being said: solutions and apps are 2 completely different things. Solutions are not moved into an app.
It's actually the opposite: a Solution can contain an App.
Also, one solution can only contain an App while the App references entities and customizations that are actually part of other solutions.
So solutions don't get migrated to Unified Interface. Unified Interface is just a way to render your customizations, by the means of model-driven apps.
If some customizations seem to be missing from your Unified Interface app, you should be able to fix that yourself by editing the app, not the solutions.
https://docs.microsoft.com/en-us/powerapps/maker/model-driven-apps/model-driven-app-overview
Sorry for the long post, hope that helps.
Henry
