I have one catalog covering files unmodified and modified covering a given time period up to the end of 2013. I have another catalog covering that same period and newer files to the present. The first has modified files that somehow were not brought forward (the files were, but not the mods) when I shifted to a new machine and subsequently added more photos for 2014 to the existing batch. I had never used XMPs to store the mods - my mistake, now corrected for the future. Is there a way to merge one catalog into another or both into a new catalog and keep the latest mods for all files? If not, do I export the modified files into a catalog and then import them into the newer of the two catalogs with all files included?
Are the older items in the newer catalogue all without modification? If so, how about importing the newer catalogue into the older catalogue but making sure that the Don't import Suspected Duplicates box is ticked?
I think I found the definitive answer, in The Adobe Photoshop Lightroom Book, the chapter on Merging Two Catalogs into One. The trick is to create a new master catalog that starts out blank, pull in the first catalog (don't move the files), then repeat it with the second catalog, which will check for and avoid dupes, again don't move the files. The final result is a combined master catalog, with the files and all modifications for all files from both catalogs. This worked for me. Thanks, you were thinking along the right lines.