According to the PSR-S650 Data List, the "BrightMidiGrand" voice is MSB 0, LSB 115, PGM 3.
Although there is no "BrightMidiGrand" voice in the PSR-S670 Data List, there is a "BalladLayer" voice which is MSB 0, LSB 115, PGM 3.
It's possible that the voice you want is still there, but has simply been renamed for some reason. It's even in the same spot in the voice list-- the 8th voice in the Piano section-- although the PSR-S670 Data List doesn't show a "Voice Number" the way the PSR-S650 Data List did.
So try the "BalladLayer" voice to see if it sounds like what you want.
If it doesn't, try the "BrightPiano" voice instead.
There is no way to export the copyright-protected voice samples from a Yamaha keyboard so they can be used in a different keyboard.