Okay, dudes. Take a step back and look at the facts. Twister's show building is much bigger than Shrek 4D, is taller and has a different layout. Shrek's queue, pre-show and main show were redone which cost quite a bit even if we take out of mind all of the temporary stuff while WWoHP was under construction (the tent/hallway of fun), add in that Shrek also butts against the employee walkways including backstage for WW and Take 5, is very unlikely that they will move into that. And, if we shoehorn Fallon in, that requires a huge IMAX screen. The showbuilding for Twister already had one of those. Shrek's roof would have to be expanded far up for that not including the prior theater wings on the backend of the building.
Remember that Shrek is literally adjacent to Potter. Whatever construction that comes with a replacement, for anything in the immediate timeframe would have to be fairly simple to meet WB/JK's contract and not impede on the land. Race Thru New York was created specifically for the Orlando resort and was done since Comcast did not like that one of their biggest assests failed to have any presence in their flagship resort. Jimmy Fallon already has presence in Hollywood on the tour, and will stay as long as Fallon stays a popular commodity. So there is no need at all for this attraction to exist at USH.
My guess as to why we have two people piping in rumors? A game of team member telephone. You won't believe how muddled these people's knowledge of Orlando is, they may work at Universal but they're no theme park experts. All it takes is for someone to mistake Twister for Shrek, and Fallon's "ride" is in "4D". You sit down in Fallon. Your seat moves. Now, Fallon =/= Shrek but you can see where the confusion may lie. So, in other words:
Team Member #1: Hey did you hear Shrek is being replaced by a Jimmy Fallon ride in Orlando?!
Team Member #2: What?! No way! So does that mean--
Team Member #1: Uh, totally. Just like we got Harry Potter, Fallon's gotta replace Shrek!
And then the legend goes on... in short, Race Thru New York at USH? Don't hedge your bets.