Edit:
I'm NOT arguing in favour of a fork. Just clarifying some facts that have been misrepresented recently.There seems to be a little misconception floating around concerning the miners and their power in the network.
"Miners are too heavily invested in SHA256 and won't allow any proof-of-work changes."
This is wrong.
But first, let's see how the miners can veto pretty much everything in the network.
The network assumes a transaction is valid and secure once it has been approved by a (or multiple) miner.
Suppose Bob sends Alice 10 BTC.
The miner receives this transaction, along with countless orders, and decides whether or not he will include it in the block he is currently making (by doing the proof-of-work).
This is how he can veto pretty much everything: he can choose to ignore some transactions for
any reason. (Which is why you should include a fee in a transaction...)
Obviously, this veto power is owned only by the miner finding a block.
If you don't or can't find a block, your decisions are inconsequent.
Let's make sure everyone understand this, because this is the important part.
In order to veto anything, you must first find a block, which is doing the proof-of-work.
Thus, as a miner, you can't prevent anyone from changing the proof-of-work; you have power only when you are finding a block.
Those with the power to change the proof-of-work are the nodes (which can be miners, but also everyone else on the network).
If, for any reason, we decided to change the proof-of-work from SHA256 to SCRYPT, upgraded nodes would drop any block made with SHA256.
Those with the powers to veto pretty much everything would then be the
new miners, those working with the new proof-of-work.
To the new investors in ASIC's: I hope you understand what it means.