Ovi was a name given to Nokia App Store similar to iStore of apple and Android market of Android.
Basically OVI Store in no way suggested that this is Nokia's store unless you know it so Nokia is making this explicit brand change by making Ovi to Nokia. NOKIA APP STORE.
In one way this is good as Nokia too has a good collection of Apps of which people were not highly aware so its better. Through rebranding many ppl will actually rething that even nokia has an app store. Apps are one of the important reasons why Apple and Android are so popular.