Sorry, “more happier” is definitely grammatically wrong ...

Some guy did a modification that might interest you.
It's almost what you are asking for, but requires switching the unit off/on to select the piano if I understand correctly.
The mod is possible without or with loosing the original toypiano sound. You will loose your warranty though I'm sure.
If you don't mind loosing the toypiano the only thing you have to do is cut a specific connection somewhere in the circuitry,

Read/watch this:

Yamaha Reface CP Acoustic Piano Sound Switch Mod

Hi Martin.

Thanks for the tip!

Yes, I have seen that video. But I'm way too much of a coward to dare putting a soldering iron inside any of my synths. Also, in the unlikely case that I would not melt some important component and ruin my keyboard, it still wouldn't really get me what I want -- a way to access the Hidden Acoustic Piano without having to turn my Reface CP off and then on.

I find it surprising that Yamaha seems so unwilling to even address this issue. After that guy found the Hidden Acoustic Piano sound, and posted his story about it in April 2017, the initial reply from Yamaha was denying it. Then, after quite a long time, and after some users had found out how to actually reproduce the action to make the Reface CP generate that sound, Yamaha eventually told us:

- - -

REFACE CP PIANO EASTER EGG
https://www.yamahasynth.com/blog/reface-cp-piano-easter-egg-1

Statement regarding the "hidden" piano in reface CP.

We are aware that many people have found an “Easter Egg” acoustic piano Voice in their reface CP.

Some background:
This particular Voice is something we use for internal testing. We use this Voice because it takes up very little space and provides an easy way for us to test various aspects of our instruments. This Voice is purely for internal testing and is not reflective of our desire to bring you incredibly high-quality, expressive instrument Voices that are perfectly matched to both the keyboard and effects.

However, if this Voice works for you as-is - and you can find a way to easily re-create - there’s no harm to your reface CP in using it. Apologies for our delay in responding.

- - -

OK, that's a bit modest and borderline self-deprecating from Yamaha. But anyway, the point here is that a lot of Reface CP customers don't really find the Toy Piano sound to be very reflective of our desire to being brought an incredibly high-quality, expressive instrument voice that is perfectly matched to both the keyboard and effects. We would much rather have that Hidden Acoustic Piano sound.

And again (I know I'm repeating myself), I don't think that we as customers are entitled to get access to this Hidden Acoustic Piano in an easier way. But it sure would be nice if there was a softwater update that would make it happen. And it would add to Yamaha's customer karma points.

But (repeating myself again), is there any way we can get this by paying for it?

Any chance getting some feedback from Yamaha on this? Anyone? Bad Mister? Anyone? Bueller? Bueller?

I haven't had the chance to try the Reface CP yet, and can't judge the quality of the hidden piano voice.
But if I read Yamaha's delayed, but official reaction I suspect the pianosample is not a high resolution, per-key multi-sampled, multi-velocity layered top quality piano, muliti GB piano sampleset. Yamaha has a good reputation to keep up being a manufacturer of both high quality acoustic instruments and digital piano's. The hidden piano sample does probably not meet these quality standards. It may be good enough and usable for a simple dance track, for song-writing on the road, or for fun. But if you need top quality expressive and dynamic pianosounds you will probably need something more advanced anyway.
I can understand Yamaha's point of view. Well, sort of ...

Makes a lot of sense.
The Piano, despite being of lower quality, also sounds better through the speakers then the sounds it was intended to make according to some people.
They probably don't want to draw attention to the speakers not being that good either.

But (repeating myself again), is there any way we can get this by paying for it?

No. Sorry.

Fair enough, Bad Mister. Any way we can get it for free?

Just kidding (smiley).

Seriously, I really appreciate your straight answer on this issue. And that's *not* being snarky or sarcastic or anything like that. I love my Reface CP.

We got what we paid for. And I'm very happy with what I got. Some of us just hoped that we could wish for a little bit more and get it ... But ...

"You can't always get what you want."
-- Some popular music group from the last century

(smiley)

All the best, Bad Mister!

Is there like a sysex message that can be sent to the unit that turns on the piano sound? I may test that by going through all the "reserved" addresses listed in the MIDI data table.