Therefore I believe that tuning the piano relative to itself but below pitch is preferable to not tuning it at all. There are many people who call themselves piano tuners who do not know how to do a pitch raise, or who will say a piano cannot be tuned up to pitch. I have not encountered any pianos that were tunable that could not be brought up to pitch. I do warn customers that there is always a chance with older pianos that strings may break. However, it is usually true that the strings will break in these pianos when the pitch is changed at all - not just from a pitch raise.
Piano wire can become brittle after many decades, and any change in tension can cause it to break.
I tell my customers that if three strings break in the middle of the piano, I will stop tuning and consult with them about the best way to procede. I do not recall ever encountering a piano on which I could not do a successful pitch raise. This is because in the top two-thirds or so of the piano, their are three strings per note. In the top half of the bass section, there are two strings per note. These are called unisons.