PDF pages can contain objects with different image resolutions. Therefore, when you have a replacement page, you should use images which are at a resolution comparable to the page that is being replaced. That is a feature of the program which creates the replacement page. It's not something you can control in Acrobat.
It would not be an issue for type or vector graphics which have no image resolution.
Thanks Steve. I guess it shouldn't be a problem then. Both of the documents would have been created by aperture and therefore be of the same resolution to start with.
One more question if I might: I'm test-driving Acrobat Pro (don't know much about it yet). I've been using Acrobat standard for simple things like adding/replacing pages. Would Acrobat standard save at the same quality of the original document or is there some loss of quality there versus using Acrobat Pro?