Is there an option somewhere that people/mods can change the question to community wiki because I can't see it, and/or does an expired bounty with auto-answer automatically move a question to community wiki?
It is no longer possible to manually convert a question to community wiki.
It's likely with your witness of the bounty situation that it was a last resort by the first owner for more attention on their question. After exhausting their efforts, and editing enough, they can also see their question switch over to community wiki mode. Bounties do not directly turn a post into wiki mode.
In the case of your question, Easiest way to clone boot partition onto new hard drive, the reason it's wiki is due only to the fact that there are more than 15 answers.
In the realm of Super User, the threshold for a question to automatically convert to community wiki is currently set at 15 answers. When hit, the question and all answers switch over.
Reasoning is that since SOFU is a Q&A empire, questions would ideally have a definitive answer. If not, and a pirate boat load of answers swans about, it's more than likely subjective and/or a discussion, and one that should not contribute reputation as it's now some popularity index instead of a correctness/problem solving issue.