Though it's not likely gonna ever happen, but if this would really happen then I would sell all of my bitcoin, because I don't think it would stay in $1M for a long time. I'm sure when this happen people will also sell their bitcoin and that makes bitcoin price drop hard.
I think that I would do the same, I think that everyone would do this because it would most likely make you a millionaire, I think that this is also never gonna happen but thinking about is already awesome.
Yes and if it ever reaches that price then the best decision would be to sell all your coins and enjoy the huge profits and if you wish you can invest that money in real world or simply get retired from work.