The big question that all of us BD5x0 owners want answered is, did LG fix the skipping and freezing issues with almost every new disc at 1080p/24Hz? That alone would be the most compelling reason to upgrade for us.
As far as upgrading the older models to Smart TV, of course they could if they wanted to. It's all Java based. Don't get your hopes up, though. LG didn't even manage to update Netflix to 2.0 in the 5 series. I wonder if the 6 series of players has the newer interface with full search capability.
Synology DS1813+ & DX513 x2
X-Rite i1Display Pro, ArgyllCMS, DisplayCAL, madVR, HCFR
HTPC running Kodi with MPC-HC + madVR, Plex for remote streaming