MPR and Vaadin 7/8 UI class buried in add-on

How does the multiplatform runtime treat references to Vaadin 7 or 8 UI class in Vaadin 7 or 8 add-ons?

On a similar topic, how does the MPR treat Vaadin 7 or 8 UI class when it is referenced in my still Vaadin 7 or 8 Views ( in other words, I have not converted them to Flow yet )?

I know there are many cases where we should use MprUI, just a little confused if I always need to. For instance, in my case, because I needed to reference the Flow UI in the class, I used MprUI to reference the Vaadin 7 UI in a place where I still needed that logic. But I still have many many places that reference the Vaadin 7 UI, and I literally need them to be the Vaadin 7 version because they are being used with Vaadin 7 add-ons. Also, part of the purpose of the MPR is to make it so I can convert things slowly, so really don’t want to rush to convert all these UI references if I don’t have to.