The arbitration of the crossbar cannot be changed because the settings are grayed out.
How do I set an arbitration other than round-robin?
The arbitration settings of the underlying crossbar have not been brought up to the AXI Interconnect top level. As a result, a standalone crossbar must be used to work around this issue.
The existing interconnect should be replaced with a standalone AXI Crossbar IP with individual 1:1 AXI Interconnects on each slave and master interface needed on the crossbar.
This configuration will allow parameter propagation to occur (such as the complex address decode settings), but the arbitration settings will now be available in the now-standalone crossbar. The 1:1 Interconnects exist to perform any interface coupling such as data width and clock conversion.
The downside of this approach is that the user will now be responsible for managing the number of interfaces and 1:1 Interconnects connected to the crossbar. Additional features such as Designer Assistance might no longer be available.
This feature is planned to be added starting in Vivado 2016.1. If the fixed arbitration of the existing standalone AXI Crossbar is not sufficient, consider creating a Xilinx Support Request referencing this Answer Record, along with details of the arbitration required.