Regarding tone or volume, neither quality will be noticeably reduced; you see, the vast majority of a piano's sheer volume is due to the soundboard's stimulation.

* * * * * *

Although you did not specify tuning sharper than A440:
If you were to attempt to tune a real grand piano as much as a semitone sharper,  i.e., A466, then you would run the risk of breaking strings due to the increased tension.

As an example, I recall watching the tv show 'Mythbusters' wherein they tested the strength of piano wire in the small gauges corresponding to upper octaves on a piano.  The strings were rated to withstand a 300 pound force (of course, divided by the fine cross sectional area of piano wire).  True to form, they withstood a 300 pound pull force, but promptly fractured when pulled as little over as 305-310 pounds.

Cheers,

Joe

PT also makes me to buy a real grand piano. I prefer to play PT on a tuning 420 to 425 Hz, after some months playing now with this tuning my ears don't want to go back to 440 Hz, I prefer the lower tuning.

I want to buy a restored piano from the begin of the 20th century.

Is it possible to tune for such low frequencies using the standard 440Hz strings? How does it sounds, there a more distortions due the lower tension? What happens with the volume, will it be louder or reduced, influence on the sound-characteristic?

Is it necessary to change all the strings, to a thinner or thicker diameter? Who did it or can explain what happens?