Changing effect parameters

I would like to display different parameters at different times in my effect. The documentation and example code that I've seen makes it seem like you get one chance to setup the parameters, and that's it.

Is this the case?

Or, is there something I'm missing about adding or removing parameters, changing their names/values, etc after the plug-in is already in use.

Imagine an effect with different 'modes' and when you switch modes (say, by choosing from a drop down) different parameters need to be shown. Is this possible?

Thanks,
-Harold