As Terence said -- the easy thing is built-in and recommended: select the Images whose Originals you want to relocate, and relocate them to external drives. External drives are _bargains_ right now. I recommend -- seriously 😉 -- buying them in triplicate: one as your "working data" store, and two to use as back-ups (never have all three in the same physical location).
Storage is so cheap, on fact, that even though for this one set of pictures you recorded a higher resolution than you need, I still wouldn't bother "down-sizing" those Originals -- particularly if you have made _any_ changes via adjustments or metadata. Change your camera settings, and just live with a little extra data spinning around.
Here is the page in the User Manual on relocating your Originals (click from the contents listed at the top).
Note, too, the following, from the page Basic Components of Aperture:
Using referenced images in your Aperture system provides a number of substantial benefits to your photography workflow. For example, you can incorporate your existing portfolio of images into Aperture without changing the current location of the files. Importing images by reference does not result in a duplication of your image files in the Aperture library, thus saving hard disk space. You can also connect and disconnect hard disks holding your referenced images’ masters as you need them. This allows you to keep masters for less-used images offline or to make specific types of images available for editing or adjustments as needed. Using referenced images in your Aperture system lets you build a flexible image management system customized to your work style. (Emphasis mine.)
For my personal Libary, I keep 4★, 5★, Portfolio Images, and anything recorded in the last 45 days on my laptop's system drive. Every other Original is relocated to a 2 TB external drive.
I find Previews _very_ useful (both in and out of Aperture), and even rely on them as a sort of emergency back-up of last resort. I recommend you rebuild yours.
