How to change the setInterval interval whilst the movie is running?

Could anyone please tell me how I can change the number of milliseconds at which a setInterval function repeats, whilst the movie is running?

I am using setInterval to call a function which duplicates a movieclip and I need the duplication to happen over decreasing amounts of time. The problem seems to be that the setInterval function only needs to be set once, so declaring it again with a new interval value isn’t an option.

Thanks in advance for anyone who knows a way round this.