We're not sure why the filming dates have been moved but if we get to know why then we'll be sure to let you know!
A BBC source also told The Mirror : "Filming for the anniversary episode starts in two weeks and there is a real buzz about it, especially because it will be in 3D." According to the same article, Matt Smith and Jenna Louise-Coleman received their 50th anniversary episode scripts last week.@dannylavery It's starting shooting in April, I believe. Date changed after we went to press in Feb. Don't know when the readthrough is.— Tom Spilsbury (@TomSpilsbury) March 13, 2013