Is the expiration of market orders going to be a soft fork? Just have witnesses reject transactions having too long of a expiration time, if they’re producing a block. This would allow that change to be implemented immediately (before HF20).
Also, perhaps Condenser should default to (or always), setting the expiration to 28 days, right now (before HF20). Most people don’t want their orders to be long-lived anyways, so this wouldn’t be a big problem for users.
It could be done that way (soft fork) but there is really nothing urgent about this. The condenser change is a reasonable suggestion.