I saw this coming a while back to be honest. With all of the drama surrounding the line-up change and them not knowing whether or not the fans will accept them and still be interested in their records I think they have to wait to see how the new album charts and how people take to it before they decide to go around showcasing it. Also when the Change tour was announced it was almost immediately after the album was released, which was about 5 months before the tour was due to begin and since there was no word on a tour as yet it wouldn't have happened until Autumn at the earliest, since they usually allow fans a decent amount of time to buy tickets and hopefully sell out the venues, not to mention that they may have to sort out the legal issues that may arise with the old Babes before they can go touring. And if they're planning on breaking Asia and America then they're going to have to go there to promote the album and perform there too, which will no doubt be pretty time consuming.
Personally, while I would love to see 4.0 in concert I think it's a good idea to delay a tour. Should they plan on releasing an album in 2011, which they probably would want to do, but nearer the back end of the year, then that would be a good time to tour because they will have released more singles with Jade so it will feel more like a tour for the 3 current members rather than a greatest hits tour with songs they didn't necessarily sing with songs of their own thrown in and let's face it, any Sugababes concert without the number 1s performed and possibly a few other classic Babes songs just wouldn't feel right. Although it's interesting to note that I haven't heard of the Babes performing Round Round as a new line-up, they've done all the lead singles and yet not performed that.