In a direct flip, only N, X, H, O and I are the same upside down.
In a flip and rotate, S, E and D are also the same and, depending on the type of font, B and K may also be the same.
SIS
this \
SOS
There is no upside down capital A symbols in runescape.
SWIMS
I only know that x and o stay the same
The word is NOON.
They are also letters upside down: y n u o s x and z Comment: Don't think that's it. Y is not an upside-down h. If this is it, c and l are missing from the set, and you'd have to allow p as an upside-down b. Besides, the answer could involve the capital letters instead.
The word SWIMS, when written in upper case letters, is the longest word in the English language that reads the same both forward and upside-down with no reading difficulty.
For All.
It would look upside down.
NOON