Placing files from one illustrator into another is easy enough, but to scale it to a specific place would be easier if you were placing the second images into InDesign which is a layout application. If you didn't have to scale the images you would be fine, right?
You can go to the "Links" palette and choose to replace the currently placed image to find your new artwork. This would work if all your placed images/artwork were the same general size. You may wish to try this and then save each file if you need to return to these files later.
It isn't clear what you need. Do you need a final electronic file to send out? Or simply a printout of the file.
If it is only a few files, doing it manually will be the easiest option. If you have hundreds of images to place, then trying to do it in an automatic process would require the files to have some uniformity.
Good luck!