Well that's a common risk that clients have to accept, and it's often what pushes them towards larger software consultancies.

The argument is that you may not be there next year and they'd be stuck in a hole with software out of date.

You could offer to commit 4 hrs per week on retainer in the maintenance mode of the software, later down the track. That gives you a retainer and gives them predictable cost. If the required changes are bigger than the allocated time per week (or they want it sooner) you'd have an hourly rate attached with it.

I always try to build relationships that last and not quick in/out software.