Is this possible at all? Because UI isn't obvious in this case.
The only way I can find to achieve this (without resorting to scripting) is to use the Copy and Paste facilities to duplicate all the modifiers and then delete the ones that are out of sequence.
Clicking Copy followed immediately by Paste should clone all of your modifiers :
I started with a Noise followed by Generator, clicked Copy then Paste to result in Noise, Generator, Noise, Generator.
Now simply remove the ones that are no longer required - in my example, removing the top Noise and the bottom Generator leaves Genertor followed by Noise - ie, the modifiers have effectively been reversed.