Can't change preview duration

I’m using Topaz Video AI v4.08 for Windows, and I can’t change the length of a preview if I want to preview a longer duration of the movie. The preview button is dimmed out. I know it says that the preview can’t be changed, but what if I want a longer duration? Why is that so difficult to do? One way I can re-activate the preview button is by changing a parameter. The only way I can change the previews while keeping the same parameters is by moving the play head to a different position. I usually remedy the situation by advancing the play head a single frame, but this is not a suitable substitution for simply changing the duration in the preview dropdown.