You probably can upgrade at any time, but you will pay through the nose for them. Try changing the rotors to DBA slotteds - you will notice a big difference for not much money (approx $150 per rotor, plus fitting).
Part numbers as follows:
Both are available in standard slotted, or cross-drilled and slotted or the new 4000 series. refer to www.dba.com.au
for more info.