changing speed of already running motor

We are using the NXT in a research paradigm delivering food stimuli to human participants, and have utilised the NXT# thus far.

I have a quick query with regard to how it would be possible to adjust the speed of an already moving motor ? e.g. if I use nxtMotor1.Run(10, 0); how can I write another line that, without stopping the motor, that sets the speed to say 5 ?

Your expertise would be most appreciated !

Cheers,
Kristjan