Please note this is a post about my experience. Yours may be very different. I am not saying this is the best or only way to do this. I am saying this is what I figured out for me. Treat it as a starting point for your own experience and go from there.
Step 1. COMMITMENT – if you’re going to do this – do it. Commit to it. I tried once before and the biggest mistake I made then was trying to run both programs at once. If you’re going to use Lightroom and move your library there, then put away Aperture. Just work in Lightroom. Otherwise you will develop bad habits and fail to make the transition fully.
Step 2. STRATEGY – know this. Lightroom does not offer a managed library option. Instead it uses “referenced” files (an option in Aperture but mandatory in LR.) So you need to decide where to put your photos. You can leave them on the hard disk in your computer where you run LR but you will soon run out of space so I suggest an external drive. I am using a 12 TB drive so I’ll have plenty of space. Then create a master folder on that drive. It can be called “photos” or “Lightroom Master” or whatever you like. But create that folder because that is where all your images will eventually end up. HINT: Whatever strategy you decide now – be sure it’s sound. It will be a big pain in the you know what to change this later so pick a strategy and stick with it.
Step 3. BACKUP – backup everything – twice. Find your original files and back them up separately. Then use Aperture’s Vault to back up the Aperture library.
Step 4. TEST the backup. Test both of them. Make sure you can recover your images if need be.
Step 5. MOVE one of the backups off site – away from your main library. That way if you have a fire or other major problem your backups aren’t destroyed.
Step 6: PREPARE your master files for the smoothest transition possible. Go to METADATA and select Write IPTC Metadata to Original. This will make sure that Lightroom sees the metadata in the original file that it imports. You can’t import changes like color and contrast adjustments, but you can import metadata..
Step 7: MOVE your managed files. Go to RELOCATE ORIGINALS FOR PROJECT. Select the drive, subfolders, names, formats, etc and keep track of where you put them. These images will be what you import INTO Lightroom so it’s important that you know where they are.
Step 8: SELECT those images you want to move and those you don’t. This is how I’m approaching this. Since any image I edited in Aperture will no longer be in a non-destructive format once moved into Lightroom, I’m moving the finished product for any of my 5-star images and importing them as TIFFs or DNGs. I’ll move all my new projects right into Lightroom – completely bypassing Aperture. I’ll move the master files and unedited images into Lightroom a dozen projects at a time over a couple of months so it’s not such an overwhelming task. I’ll also always have them in Aperture so if something urgent comes up, I can go back to Aperture, find the master file, export it from Aperture, then import it into Lightroom. It’s like I have backups of my backups of my backups. You don’t have to move everything and you don’t have to do it at once.
Step 9: Organize Lightroom as closely as you can the same way you organized Aperture once you bring your photos in to LR. This will delete the pain points associated with a whole new system.
Step 10: Be ready to start over. I had to do this three times before I figured out a correct way for me to live in the Lightroom universe. For that reason, I suggest you start small – experiment a while, and decide if you need to make any changes.
The concepts are all there, it’s just the differences in image management and learning new shortcuts that take the most time. So far, I have no regrets about making the switch except for some details I am still working out.
Good luck and have fun.