You mean something like a conditional enablement of an expression? This is a new feature in QV11, so if you are using QV11, you might want to look into the Whats New in QV11 demo, especially the Reports tab:
If you are using QV11 there is a easier way. Look into the last sheet in whats new in qv 11 file (available in the installation directory).
If you are have different version this can be done thru macro (I used to use this but not anymore). APIs are available to remove the existing expressions and add new ones. Create a inline load with list of fields you need to use and trigger this macro when the field selection is changed.