With everything that is currently out there you can use the older buddy boxes with no problems. At least with the first releases of 2.4 GHz radios all most of the the manufacturers did was update the transmission circuits inside the radio, but the rest of the functioning of the radio is identical to it's 72 MHz counterparts. So yes, you can use your old buddy boxes.

Spektrum 2.4 can use a Spektrum or JR buddy box
Airtronics RDS8000 can use any Airtronics but the Quasar
Futaba can use any Futaba that you have the matching buddy box cord for
JR can use JR or Spektrum

Ken