Think about it, a hard fork can influence the BTC price in only two directions:
1) everything runs fine and bitcoin price will continue to vary sideways.
2) the hard fork causes problems and BTC price plummets down to zero
So you see it's either sideways or rock bottom! There's no possible scenario where the hard fork will pump BTC price. It's not like people will think "omg block size increased, let's all buy bitcoin!".
So if you sell now, you're protected from a rock bottom price drop.
If the transition runs smoothly, you can buy back in later.
Sell now to protect you against the remote possibility of the fork ruining bitcoin.
Simple fork wont ruin bitcoin, and the very idea is insane.
There is no way to predict how and will the price even change, but im guessing it will go up 10% for a short while after forking.
Block size increase is important, well better said will be important later as the number of transactions increases.
Move along folks, nothing to see here ..