1 Set the Master / FX / 'Setup' to: Multimode Effects Control: preset on MIDI channel 16 (you can use any channel you want).
2 Create 2-new presets from Preset Manage / New. These presets will be used to assign Preset FX settings only ... THERE WILL BE NO VOICES ASSIGNED TO THEM, THEY CONTAIN NO SOUNDS.
3 Edit the first preset (Preset Edit). Choose 'Global' & 'Edit All'. Under the global preset parameters select 'Preset Effect A' and choose Warm Room: Main FX A Send: 20%
4 Edit the second preset in the same way EXCEPT set the Main FX A Send: 50%.
5 Assign the first preset to Midi Channel 16... play the sequence. Somewhere in the sequence send a program change messege to channel 16 and assign the second preset that was just created. You will notice that when you assign the second preset the reverb mix reflects the new 50% level.

Note: This is a basic setup...you can create as many 'FX Presets' as you want with various settings. By sending a program change messege to channel 16 (in this example) you will be able to then 'automate' FX changes in your sequence. HINT: Try this with various settings. Choose a delay and change the Feedback or Delay time. Try changing the Preset Effects A 'FxB==>FxA' amounts. Try using the same reverb but change the 'Decay Time' and/or HF Damping.