CJam, 16 bytes
qA,'G,65>+s_W%er
This is fairly long because CJam handles base changes differently, so it was shorter to just do transliteration. See my Retina answerRetina answer for more on transliteration.
Explanation
q e# Get the input
A, e# Push [0 1 ... 8 9]
'G,65> e# Push "ABCDEF"
+s e# Combine and convert to string
_W% e# Make a copy and reverse it
er e# Replace each character in the first string with
e# the corresponding character in the second