Actually, leaving it alone is the option that changes the rules the least. Putting a lifetime on coins is a huge change, regardless of what happens when they expire.
It seems possible to me that miners might just decide to delete old parts of the block chain. If miners tend not to accept old coins, then other miners might decide not to risk including them.
Worryingly, this is actually a soft fork, since you are not adding a new feature to the network. You are making the network more strict.