Is there a function/button/etc to allow me to select the large image in the document, and tell Inkscape to replace it with the smaller one?
Assuming you didn't use the embedding option when you imported the image, right-click the image you want to replace and choose
Image Properties... from the flyout menu. In the
Object attributes dialog box that shows up, enter the path and filename of the smaller image in the
URL field. The result is instant.
Bear in mind, however, that the smaller image will follow the dimensions of the larger image. So if the smaller image doesn't follow the same height and width proportions of the larger one, the result will be stretched either vertically or horizontally.