The change in production schedule just seems like such a major move that they must have a very good reason for it. I'd really expected most of the production team and Tennant to take off after S4; if they're going easy for a year, it seems like it's being done to give all of them a break, which sort of implies that they'll return afterward. If there's going to be a new Doctor and a new producer for S5, why not just throw in the towel at the end of S4 proper? But whatever's going to happen, I'm sure it's planned. It's too big a shift for them not to have it completely worked out.