Photomerge as Video

Hello Community!

I am producing some photomerged images as part of a Then and Now series for the city I am based in and my question is:How do I do this as a video?

I would like to make a clip where the historic imade blends slowly onto/into the new image. For example, a building destroyed in World War 2 blends onto the present day image of the same location. I'm hoping it will be something quite simple such as using the layers created in photoshop and applying them into Premier Pro or After Effects... Any ideas?

Many thanks!