Not sure about any migration documentations, but if you are running HSRP or VRRP between your existing 6513 switches. You can replace the VRRP or HSRP stand-by with a 4510 and make sure to bring it up as stand-by. Once that is done, you can change the priority on the new 4500 so it become the active switch and the other 6500 stand-by. Now you can replace the other 6500 with 4500 and bring it up as stand-by.
Warning: you need to have an outage window to do any of these procedures, as you can never know what may go wrong.
HTH