It's very simple to toggle specific effects on or off while the game is running. Just find the specific toggle key option in the preset settings and set it to some key code, i.e. "FXAA_ToggleKey", "Bloom_ToggleKey" ...
Switching entire presets without tabbing out of the game is currently only possible with the SweetFX shaders, using the Overwolf SweetFX Preset Selector app:
store.overwolf.com/app/aRestless-SweetFX-Preset-Selector
.