# 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 answer](http://codegolf.stackexchange.com/a/68308/29750) for more on transliteration.

[Try it online.](http://cjam.aditsu.net/#code=qA%2C'G%2C65%3E%2Bs_W%25er&input=%23A1B2C3)

### 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