The netlist in PADS is based on pin-pairs. A net with just two pins has one pin-pair. A net with three pins has two pin-pairs, and so on. (This is why you can't have single pin nets.)
You can assign a pair of nets as a differential pair that applies to all the pin-pairs of the net, or you can assign the individual pin-pair pairs separately.
The connections, or ratsnest, or unroutes, or whatever you want to call them, determine how the router works. Each of these represents an unrouted pin-pair.
The topology of the nets determines how the order of the connections is treated. (Setup > Design Rules)
Minimized is the default, and means you can route any pin of the net to any other pin or trace.
Serial, parallel and mid-driven are topogies for specific kinds of nets that need to be routed in a specific order. Almost all differential pairs are serial.
So when there's just two pins of a net, all the topologies are the same. When there's more, then you need to setup the pin-pairs to route in the correct order.
This is what the Reschedule command in Router is for. You can select the unroute, then the reschedule command, and are then presented with pins that are valid new destinations. Once they're in the correct order, you set the net to be protected, so no matter how the components are moved around, the routing order is maintained.
So my suggestion was based on the assumption your diff. pairs are just a single pin-pair for each net. My solution to route the net with two sets of rules was to break it up into two pin-pairs. A new part placed where the transition from one rule set to the other needs to be. You assign one pin-pair with one set of rules, and the other pin-pair with the other set.