Anti-transaction replay in a hardfork

The rationality of actors in this situation essentially defines the limited solution that is possible. Upgraded software can create transactions guaranteed not to execute to one fork or the other, or that is not prevented from execution on either fork. This would result in much less chaotic markets and well-determined prices for the two currencies. However, none of this precludes the possibility of further forks on either side, and the difficulty consideration alone suggests a likely counter-fork by (part of) the existing network. Mining is a random process and the hashing power is going up and down which adds to the complexity of the situation.