Differential Pair Routing feature has been improved to enable easier width and spacing changes on the fly. The Design Rules work with the interactive routing to enable width and space to maintain the differential impedance of the route, while performing operations such as neck-down to BGA, and escape routing.
Before now, changing the differential pair gap during routing did not automatically change corresponding trace widths.
This meant the differential impedance was not maintained when performing neck downs, and in some cases layer changes.
In Altium Designer 14, the width and gap of the pairs are bound together in the Differential Pair Design Rules, so adjusting one will automatically adjust the other.
So you can have confidence that the differential impedance of the pair is accurately maintained across the entire board.
Even when necking down into a BGA fanout.