The older 'standard' appears to be a French one at 435Hz.
While, 432Hz was used by Verdi; also referred as 'Verdi Tuning',
based on a power of 2 calibration for middle C.
The ISO 16 standard finalized to 440Hz (among other variations;
including 439Hz being a prime and hence not easily reproduced in the lab).
So, I guess it is not quite like 432Hz was changed to 440Hz
(you could probably say 435 was changed to 440... yet, there is a lot of detail on Wikipedia about this).
Extracts from the Wikipedia page on Concert Pitch.
The A above middle C is often set at 440 Hz although other frequencies
are also used, such as 442 Hz and 443 Hz. Historically, this A has
been tuned to a variety of higher and lower pitches.
And, there is a lot of other interesting information there and on linked pages about its variations.
The most vocal opponents of the upward tendency in pitch were singers,
who complained that it was putting a strain on their voices. Largely
due to their protests, the French government passed a law on February
16, 1859 which set the A above middle C at 435 Hz. This was the
first attempt to standardize pitch on such a scale, and was known as
the diapason normal.
and after that,
The diapason normal resulted in middle C being tuned at approximately
258.65 Hz. An alternative pitch standard known as philosophical or scientific pitch, fixed middle C at 256 Hz (that is, 28 Hz),
which resulted in the A above it being approximately 430.54 Hz.
The appeal of this system was its mathematical idealism (the
frequencies of all the Cs being powers of two). This system never
received the same official recognition as the French A = 435 Hz and
was not widely used. In recent years, this tuning has been revived,
sometimes as 432 Hz under the name "Verdi tuning" since Italian
composer Giuseppe Verdi had proposed a slight lowering of the French