If the user can feasibly switch between Registered & Declined in real-life terms, you probably want to be setting those Progression Statuses to have the same step. Then in your Marketo Program, the lead can switch between these without restriction.
To do this, go to Admin > Tags > Channel and edit the Channel that your Program uses. Then simply change the step numbers of Declined & Registered to be equal.
Let me know if that does the trick!