Storage needs grow, and I have had to juggle quite a bit of hard drives and recently added a full set of new 5TB drives, and it has become a NIGHTMARE in a sense....
But LR is so slow with 3 factors in play...The number of drive to the number of images in my catalog and the size of my files from 40-50mpixel raws.
So if you have an old catalog working and you are wondering to merge. I say just leave it, or get an old hard drive and dump the content on it as a back up. But if you have a faily smaller size number of images total, it wouldn't be so bad to put them all in one catalogue. It will upconvert to the new version if from an older one, and it will ask before hand. LR does a lot things great. But the speed has had me move to alternate products as its unusable. Which reminds me...always use a folder structure you can revert to, and not something the catalogu is dictating where you files are. Meaning , dont rely solely on LR for keyword look up unless its a universal location for the search.