I just imported all my pictures to create a new catalog after upgrading to LR6. Can I delete all the other preview files? I have never really understood why I have so many preview file folders...
You probably ought to keep the old previews, and upgrade the old LR catalog to LR6 (File->Open Catalog, point to the old catalog file), and once that is successfuly, delete the earlier LR6 catalog and its previews, and possibly the old LR catalog and its previews.
Thanks for your response, dj_paige! Just out of curiosity, I moved all the old preview folders (there were 9 of them!) to another folder, and opened LR with the new LR6 previews. Worked fine. I then looked at the 'properties' of the combined old previews - 9 folders going back to LR2 totalling 152,582 files and 44.1 GB; the new LR6 preview folder has 82,157 files at 2.03 GB. Not really understanding how Adobe structured all that, it looks like a lot of duplication - the old number of files was double the new. I noticed too that the new preview folder is not labeled as Lightroom 6 - 'Lightroom Catalog Previews.lrdata', unlike the previous versions. Am I missing something - is there something else in those old preview folders that I need to keep? I am thinking I could clear up about 44GB on my hard drive!
Previews for catalogs you are no longer using can be deleted.