A couple of months ago:

http://www.businessinsider.com/if-trump-quit-presidential-race-what-would-happen-2016-8

That means that if the RNC replaces Trump, his name could still appear on the ballot in some states — and not the new nominee selected by the party. This could be incredibly confusing for voters who would want to vote for Pence, but would have to select Trump on the ballot, political scientist Josh Putnam told The Washington Post.

Several ballot deadlines have already passed, and more are coming up this week.

"The deadline for things not being messy is like now," Aull said. "By the end of the month at the latest, that's the non-messy deadline, whereas post-August or end of September is when things start to get really, really complicated. They don’t want that."

Another state-specific hurdle would come up after the election.

When voters select a candidate, they are really telling members of the electoral college in their state to vote for that candidate. In some states, the electors can choose whomever they want for president. Others require they vote for a party based on popular vote. And in a third set, electors are legally bound to vote for the name on the ballot.

It's this third category where the Republican Party would have to go to court to transfer the votes for Trump to the replacement. That would be even more time consuming and "messy."

What if the party replaces Trump too late?
If Republicans win states where the filing deadlines have passed, the RNC would likely have to appeal to the courts in order to give the new nominee the votes cast for Trump, Aull said.

That scenario would obviously be a hurdle for the Republicans, so if the party does decide to replace Trump, it would be better off doing it as soon as possible.