Obviously there is a lot of material available from 3e and I own most of it in PDF (and still have my CS in hard copy). Is there anything major to look out for that has been changed or retconned between 3e and 5e? Like, the players won't likely know or care but I would like to use RftLW as my foundation and I don't want to go research something more deeply from 3e only to realize it doesn't fit.
From what I know, the biggest thing change new to 5e would the political environment in the Mror Holds. In 3e and 4e, there wasn't much going in the Mror Holds outside of the usual politicking. There was the ancestral kingdom of the dwarves deep below the surface to explore, but there wasn't much of anything down there. So if your party wasn't interesting in dwarvish politics or in dungeon diving, there wasn't that much reason to pay attention to the Mror Holds.
In 5e, that ancestral home, now known as the "Realm Below", is now a battlefield. Instead of empty ruins of a long dead dwarvish civilization, it is now an alien world inhabited by the Daelkyr and their abberations that laid low the forebears of the Mror dwarves. The dwarvish clans are reacting to the Daelkyr threat in different ways; while they all want to keep the Daelkyr contained below the surface, there is much argument on whether to purge every aspect of their corruption from the world or to research and reverse-engineer it, to turn their own power against them.
Adventuring teams that choose into explore the Realm Below now have the much more immediate and obvious goal of holding back the Daelkyr and possibly recovering their symbiont technology, as opposed to the vague goal of unearthing ancient dwarvish relics. Not to say that adventurer's don't find dwarvish heirlooms and treasure in the Realm Below; they still do, but that's not the only reason why they're there anymore.
Of course, if your table never goes to the Mror Holds or otherwise engages with the Daelkyr, that might not be that big a change for your group.