No rule changes. Trump is not the anointed one. Let the GOP rank and file decide whether to support an opponent to the President's re-nomination.
Clowns come along, in forms like John Anderson, R-Il who challenged Reagan.
By the end, this Anderson guy supported Ralph Nader, joining other clowns to follow.
2016 witnessed the 16 Dwarfs.
It would be fitting, for other Republicans to oppose Trump, since Republicans today are the most stupid, ineffective, deranged bunch I have witness in my lifetime.
One proof of their stupidity, is blocking/failing to enact conservative measures, solely because it came under Trump.
I would advise Trump NOW to run in 2020 as an Independent. Criticize democrats, and laugh at the lousy performance on the GOP when they held the majority and the power to YET FAILED to enact Obamacare Repeal/replace and FAILED to enact stronger borders and illegal immigration control.