I would like to make the second version closed source. This answer suggests that in order to do so, I would need the contributor to agree. If so, what form would this agreement need to take? Would verbal agreement or an email be ok or can I just update the licence on my repo and make it private as Martijn's answer seems to suggest? I realise that the original code can still be used under the MIT license so someone coming across the forked code could use that.
To sum up, can I just switch my project to private (and update the license) otherwise which steps must I take to get the agreement of my one contributor, make the repo private and any further versions closed source?
UPDATE: I just came across this answer that seems to suggest I can just make my repo private.