Hi,
In short, no you can't change the idle thrust (either ground or flight). The ground idle thrust is set as low as possible during the design phase to reduce brake wear while still maintaining a stable flow through the compressor. The flight idle thrust is set to ensure the engine can accelerate to MTO in a short time to allow an aircraft execute a go around etc..
In general the Trent's have a lower idle thrust than their GE / PW counterparts but we're not talking massive numbers.
All the best,
P2C