Ok, I am currently looking at the older and newer versions of GAF. They are sitting right here in front of me, and I can tell you that the text on them is different.
GAF: PSV-094
When this card attacks, it is changed to Defense Position at the end of the Battle Phase. This position cannot be changed during your next turn.
GAF: DB1-EN202
When this card attacks, it is changed to Defense Position at the end of the Battle Phase. YOU cannot change this Battle Position until the end of your next turn.
That is the exact text on each card. Now, if you read carefully, you will see that both cards allow GAF to be changed if he is targeted with Change of Heart.
GAF

SV-094 allows it because it states "during your next turn". So if your opponent takes it in his turn, which is NOT your next turn, he can switch it to Attack Position and attack. It is then turned to Defense Postion, and given back to you at the end of his turn. Now that you are in "your next turn", you can't change it back and attack because of the effect. Also, just for the record, if your oppponent somehow(say, Snatch Steal) gets GAF back again on the turn after he attacked via Change of Heart, he cannot attack either because it is now effectively "his next turn".
GAF

B1-EN202 allows it because it clearly states "YOU", meaning the owner of the card and/or the one who activated his effect by attacking. So if "I "attack then "I" activate his effect, and it only applies to me, not my opponent. So he can Snatch, Change, etc. and attack with him freely.
So, to answer this question correctly, Yes GAF can be changed by your opponent after you have attacked.
Your welcome,
TooNz
"Not all who wander are lost." J.R.R. Tolkien