http://moviehole.net/201371331fast-furious-7-delayed-to-april-10-2015

Vin Diesel has announced – not unexpectedly, with Paul Walker’s tragic passing complicating the filming – that “Fast & Furious 7″ has been pushed back from July to April 10, 2015. That’ll give director James Wan, as well as the film’s producers and writers, time to retool the script (they’re trying to ‘retire’ Walker’s character from the film, with scenes already shot for the film).

Why? You'd think the fans would be happy that Walker is still in the next film because they had already shot it with him, right?

Because it was only half done, I would presume, so not all the scenes were finished.

Exactly that ^

Not all his scenes were finished, so they are retooling the script and his brother is actually going to finish the movie for him. They will try not to do too many closeups as they are going to CGI the ones they have to do to make it possible to do this film. His brother is agreeing only because they are going to find a way thats respectful both to the family and the franchise to take Brian O Connor out of the series