The system doesn't actually make any distinction on whether the previous Reputation you issued out to someone was an Approval or a Disapproval. The system could stop you from Approving someone because you already Disapproved them too recently, and vice versa. All the system cares about (for purposes of "spreading it around") is whether or not you've recently
touched the same person's Reputation.
As for if/how/why/when you can Disapprove, nobody loses that capability for forever (unless they are banned or they've done something else almost-as-seriously wrong). It will eventually come back to you.