Good to finally see an update to the 6-series. Gee, it's only been about 5 years! But, yeah, it really seems more like a refresh than a really new model -- kind of like what happened to the 7 and 9 series when Yamaha went from the PSR-S770 and S970 to the S775 and S975. And $1400 list price? Are you kidding? Sure, the street price will probably be -- and this is just a guess -- $1000-1100, but that's still pretty high when you consider that the current model usually can be bought for about $800 or so (all prices I'm mentioning are in US dollars). And when I look at the S670 and SX600 side by side on Yamaha's website, it does indeed look like that they use the same shell/housing for both keyboards.
Now, how about the E400 series, Yamaha? That model is past due for an update, as well. They put the PSR-I500 on the US website, which is good, but that is really just an Indian-music focused version of the PSR-E463 -- also appearing to use the same shell/housing as the E463 -- where the DJ groove patterns of the E463 are replaced with Indian music specific features, and a few other features are added, such as portamento and additional settings for the sustain pedal.