I never switch media halfway through unless I see a big problem developing with the picture and think that switching to a different medium is the solution.
I guess I'll sometimes start a picture, abandon it halfway through, and then come back to it months or years later and think "this would be better in pencil crayon". But at that point it's more like restarting than switching.
Hmm, yeah that makes sense. I've had a few where I leave for months (or years) to 'age' and then come back and say, "I should totally remake this...with something else." By then it does feel fresh, restarting as you say. *nod*
I tend to go with what I think would look right for the piece, so yes I switch up quite a lot, I've even restarted because something didn't work.
From personal experience if I am trying to do a watercolour/digital etc and just keep pushing and pushing til it fits the fit is never right, so it's best just to change your plan than keep ahead with one that isn't working.