UWP is a different beast than a regular NET app so you cannot simply "migrate" from your codebase to UWP. Copying DLLs will probably result in errors about them not supporting the runtime. You must use assemblies/packages that support UWP.
I would recommend you follow the instructions given here for creating a new Unity UWP app. Then bring across your Unity-specific code piece by piece and adding the UWP-compatible packages to get it to compile. You'll undoubtedly need to make code changes because the runtime model of UWP is different than what you're using now but hopefully the changes won't be too bad. Again, do this piecemeal until everything is converted so you can fix compiler errors as you go along.
Further support for Unity should be posted in their support channel.