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.
people will start selling it much lower than this even when price hir $1000.Unless the price reach $1m overnight,nobody will wait to see price hit $1m as they know that bitcoin price can fall anytime