Is it possible to upgrade the samples on a P120?
Unless someone has hacked the system and burnt new ROM chips with
different samples (which is highly unlikely) no.
If it's got MIDI connectors, you can just run its MIDI OUT to the MIDI
IN of anything that's got a piano sound you like, and go from there to
an amplifier. I've messed around with a P70 this way - used its
keyboard to drive an external box that had a piano (and other sounds),
and also used another keyboard to drive the sounds in the P70. I've got
a P85 now (newer version of the P70) and I'm sure the same is possible
with that - am not familiar with a P120.
If you really want to upgrade what's in there, best to contact Yamaha
via their web site and ask them directly.
-S-