Upgraded from LR4 to LR5.7. Apparently successful download on iMac27" 5K running OS X10.2. Collections that were clear in LR4 appear badly blurred when opened in LR5.7. What's,wrong?

Question

Upgraded from LR4 to LR5.7. Apparently successful download on iMac27" 5K running OS X10.2. Collections that were clear in LR4 appear badly blurred when opened in LR5.7.  What's wrong?

Answer 1

Thanks, everyone, for your suggestions.
John: I back up the catalogue (and optimize/check its integrity) every time I quit Lightroom - which is why excluding it from Time Machine isn't the end of the world. I did rename the catalogue early on in my troubleshooting process - as you say, I didn't like the fact I felt out of control of it.
I tried synchronizing a folder that had some flags: when I'd done there were three flags in the folder. (I have unclicked - and clicked and then unclicked - the 'unsaved metadata' checkbox in View Options: as I say, Lightroom shouldn't be displaying these flags at all.)
Rob: Thanks for understanding why I'm fretting! I've checked my XMP sidecars, as you suggest, and they have read and write privileges. I then checked the last-modified date on the XMPs that came up with flags after I'd synchronized that folder. One of them displayed the time I'd just synchronized the folder (it's one I haven't worked on apart from applying a blanket preset in LR2, and then a blanket update to PV2012 in LR4, as I've done to all the images I haven't yet worked on individually); the other two showed the time (I presume) I worked on them yesterday. I selected these three and applied command-S: they then showed, correctly, that they'd been modified a minute before. So far, that folder's now flag-free. (As I said in my original post, the flags sometimes do seem to vanish if I save enough times - and they're unpredictable, not necessarily appearing on the same images in a collection or folder as before I do I group save/clear conflict.)
Mollysnoot: I don't see how the Mac OS would be doing this (although who knows - I'm not a software expert). I don't have any other programs open that can modify images. And since these flags can reappear instantly when I've dealt with the conflict - before I've had time to take a breath, even - I don't think they represent any actual modifications.