I have come across many old pianos that are a semitone or more flat. The piano loses a lot of volume and tone; piano strings tend to sound best when they are at much higher tension. Many old pianos will survive being raised to A 440 Hz but alas some will not. Raising the tension on old frames is always a risk but is usually successful.
I have an old piano and it is very flat, should I get it raised to pitch?
Many older instruments can indeed be raised to A 440 Hz but there are some things to be aware of:
The best way is to have your piano tuned regularly
Pianos like to be tuned regularly. If your piano is played a lot this might be every 6 months, if it is played less every year is a good idea. Why? There is approx. 20 tons of tension on the piano frame. If the piano is very flat or sharp a change to correct pitch over the whole piano will add or subtract a lot of tension on that frame! This makes strings stretch all over the place, usually unevenly and results in an unstable tune.