Hello,  there seems to be confusion about A=435 Hz and A=440 Hz tuning.
The 1937 international symposium on tuning adopted as standard 435 Hz
at 15 degrees centigrade.  This had been used by the majority of
builders (but not all) for some years.  This is very close to 440 Hz
at 20 degrees centigrade, so you can see that there is not a great deal
of difference given today's warmer buildings.

The main reason that xylophones can play at 440 Hz alongside an
instrument at 435 Hz is the human ear.  Pitch discrimination is only
apparent in the middle ranges, not with higher or lower notes.  Also,
xylophones give off an awful lot of overtones, partial tones and odd
harmonics, which together tend to mask the correct note, therefore it
becomes unnoticeable.

However it is better to stick to 435 Hz to be on the safe side.  To
re-tune a full size concert grand from 435 Hz to 440 Hz adds six tons
of extra tension to the frame, according to the old Grotrian-Steinweg
firm.  Raising the pitch is not a brilliant idea given the uncertainty
of some of the old castings.  This has resulted in many a good stringed
instrument being ruined.  It is almost impossible to let the strings
down again and keep the instrument in tune.  It tends to vary, and
require constant re-tuning, after the strings have been over-stretched.
The only real solution is to re-string the instrument -- very costly.

Many old instruments, both string and pipe, have tunings and
temperaments far from today's standard.  If you don't understand it,
leave it alone and consult someone who does.  The worst cases are
organs -- un-tutored people shorten or mutilate pipework, to make them
fit modern pitch and equal temperament, making it very difficult to
restore correctly.

Hope this helps a little,
John Harrold