Well...
It's really icky to punish someone for doing something that wasn't a crime at the time and place it was done. If you create a law that prohibits visiting a park at night, you shouldn't retroactively punish half the population for having visited a park at night. This stuff is UN human rights stuff, and often the norm in sensible justice systems.
Drawing on this, it's not currently illegal to publish a warrant canary. If it's not illegal, you can't be punished for it now. If you can't be punished for it now, in a sensible justice system, you can't be punished for it down the road either, as long as you stop doing it before it becomes illegal.
What they could argue is that by ceasing to publish the canary, you're committing the crime of communicating something you're not allowed to communicate. That, however, skirts dangerously close to forcing someone to lie.