I'd imagine one would move just because of the theater owners alone. How would decide and allocate screens for each movie, 2D, 3D and IMAX?
For IMAX, it's contingent on the agreement the studios sign (usually it's first come first serve) -- and how long those screens belong to that movie. When WB moved
Harry Potter and the Half-Blood Prince from November 21, 2008 to July 15, 2009, IMAX couldn't play the movie until July 31st because of Paramount's exclusive agreement for
Transformers: Revenge of the Fallen. Yet if a movie is scorchingly popular and demand is high for the premium format... sometimes IMAX will split the showtimes between two movies. IMAX was legally obligated to show
Dark Shadows day-and-date with regular theaters per WB's agreement, but due to
The Avengers' popularity -- DS's showtimes were halved to accommodate the demand for the latter. So if WB is the first to make a deal with IMAX for a DMR version of BvS on May 6, Disney will have a harder time getting those screens for the same weekend. (Or several consecutive ones.)
I also remember reading a DHD article by Nikki Finke saying that Paramount also had a hold on certain 3D screens for
Transformers 3, which conveniently ended right when
Captain America hit theaters... so they kept the hold on the 3D screens for another movie they distributed.
So if WB and Disney stay put, it will be a scheduling headache for theater exhibitors -- IMAX and multiplexes alike. Moving either film up a week or more will greatly alleviate that aggravation and satisfy seat demand.