A lot of things that have been happening lately, like Chris not being a focus in recent Glee promo in favor of DaleaStreet (gag) and the total lack of mention of Kurt spoilers for season six (if there is a season 6 at this point). I'm betting that Chris signed on for this project a long time ago and his team has been in active discussion with the Glee management about Chris having time to be away to work on the film. If production is set to begin at the end of the summer, that would have prevented Chris from being available for at least the first few episodes. My guess is that with the time jump, they would have planned that Kurt was either busy working on a show or away working on a film (yay meta!) to explain his absence or limited screen presence.
But now that season six is up in the air and there are serious questions about how many episodes we'll actually get and when Glee production might actually start, how much filming Chris might miss is an open question. If Glee isn't coming back until 2015 and the whole premise needs to be reconsidered and likely rewritten in order to accommodate a more limited season then production might be pushed back until after Chris is done with filming his movie and can be on the Glee set full time.
Regardless, it appears that this is only the first of Chris's film projects. He's clearly moved beyond Glee and if his presence on the show is more limited because he's working on outside projects and not because of RIB favoritism, then I'll be well satisfied.