CNN has announced that they will air Trump's rally that he is holding live opposite the Fox News Debate in a bit of a shameless ratings play. The thinking here is that this might cut into Fox News ratings, but let's be honest: the people who were only tuning in to watch Trump were not going to watch tomorrow night anyway. The fact that he will be on CNN will not really change that equation.
However, if you are the sort of person who finds Trump's stream-of-mostly-erroneous-consciousness to be entertaining for 2 hours at a time, then CNN has got some alternate entertainment planned for you.
None of this will change the calculus of how this will play in Iowa, however. Trump's most loyal supporters will of course say that whatever Trump does is the right thing because Trump did it. However, not everyone who was potentially planning to vote for Trump is a die hard Trump supporter; Trump has fence-sitters in his camp just like every other candidate. And Iowans have grown accustomed to their pecking order in the national discourse, and guard it jealously.
As silly as it seems in 2016, they really do expect you to come to their little breakfast gatherings and flip their damn pancakes and come to their ag fairs. Thursday's debate, as the final debate before Iowa, is one of the most major events to happen in Iowa all year, and is part of the process of kissing the ring of Iowa voters. Trump will doubtless pay some price for refusing to do it, although how much remains to be seen.