Have you seen these support articles?
If you are using the most current versions of iPhoto and and Aperture, you can update your iPhoto Library in iPhoto and open the iPhoto Library in Aperture and use it like your Aperture Library. Both Aperture and iPhoto share a unified Library format.
If you want to turn the iPhoto Library permanently into an Aperture Library, import it into an Aperture Library (File > Import > Library). You can then delete your iPhoto Library after making sure the import went well and you have a working backup of the iPhoto Library.
But I would not recommend to delete the iPhoto application. Keep it. The size is less than 2 GB, and you may need it for trouble shooting. For example, if you have hidden images in iPhoto, you cannot unhide them in Aperture. Also, Aperture cannot edit some iPhoto library items like Slideshows and Books, cards, calendars. You still cannot create cards or calendars in Aperture.
If you have both, Aperture and iPhoto, you can browse two libraries at the same time. And iPhoto can reveal managed original files, what Aperture cannot do. Aperture will not beable to open erlier versions of iPhoto libraries from backup.