You can download old perpetual versions of Lightroom 4, 5 and 6 on this Adobe page.
If you have a perpetual (stand-alone) version of Lightroom 6, download and run the Mac or Windows application installer, and then download and run the Mac or Windows 6.12 update (or 6.13 if it is out by the time you read this). You don’t need to download and run the earlier updates.
If you’re reverting to Lightroom 4 or 5, there will just be one file to download and run.
If The Old Version Doesn’t Find Your Lightroom Catalog from that Version
If when you install your old version of Lightroom it can’t find your old Lightroom catalog, then choose Choose a Different Catalog, and navigate to and select your catalog file (.lrcat file). If you don’t know where your catalog is, then it’s most likely in a Lightroom folder in your Pictures folder. If you get a warning that the catalog is too new to open, you have chosen the Classic 7 one rather than the catalog for your old version. If you get a warning that the catalog needs to be upgraded, then you have chosen a catalog from an older version than the one you’re installing.
If Lightroom opens to a blank catalog – no photos – then go to File>Open and navigate to and select your catalog.
Along with your catalog file (.lrcat), Lightroom creates a Previews file with JPEGs of all your images. When you upgraded to Classic 7, the installation took your Lightroom 6 Previews file and changed its name to correspond to your Classic 7 catalog. When you open up Lightroom 6 for the first time after reinstalling it, Lightroom 6 will start to regenerate previews for all your images. You can just let this run, or instead you can recover the Classic 7 previews file as follows:
- Close Lightroom 6
- Navigate to your catalog folder
- Identify the Classic 7 “Previews” file
- Rename it to “(lr 6 catalog name) Previews.lrdata”, where you replace (lr 6 catalog name) with the name of your Lightroom 6 catalog.
- Reopen Lightroom.
Users of earlier versions: I don’t believe that Classic 7 takes over Lightroom 5 or earlier Previews files – but if your Previews file starts out very small after reinstallation, in comparison to the Classic 7 Previews file, you can also rename the Previews file as described above.