To answer your question (assuming another retailer is available), then you can simply watch the film using them. That's the great part about UltraViolet (UV), you can watch how you want and where you want. For example if you register with Flixster and link it to your UltraViolet account, you'll most likely be able to watch your movies there too, I already do this.
UV is still in it's infancy and growing. Did you know that Barnes and Nobles upcoming Video platform for Nook will support UV? So that should mean that just like you can watch your movies via Vudu or Flixster, you should be able to in the future watch them using Barnes and Noble Nook video apps, which should be available on almost every platform like the amazon's kindle apps.
To address your desire to download them, the different retailers provide there own unique downloads that work for offline watching. However, in the coming year UV is releasing a standardized video format called "Common File Format (CFF)" for video files. These files are created to an approved specification much like a DVD or Blu-ray disk is made. New devices will become available that are UV certified. Anything that is UV certified will play these files. So I would expect big things in the future, but as of now it's still finding it's way.
Also a really cool thing with UV, is that you can add other users to your UV account. They get there own UV login and can then setup there own Vudu account and link to the UV account you created for them and this allows them to stream your movies also. This works for up to 5 additional users. It's like owning 6 copies of the movie or TV series that you only purchased one copy!! I think that is one of the best features! However, you can only have 3 people streaming simultaneously..
My only gripes with UV is that Disney and Apple need to join the DECE. But if UV continues to grow, I would think Disney would join in the future, possibly apple also.