Upvote:1
Three principles:
You may adjust your itinerary after the visa was issued.
You may not misrepresent your itinerary before the visa is issued.
You should not give the false impression that you misrepresented your itinerary before the visa was issued by gross changes after it was issued, even if the changes were genuinely afterwards.
For instance, if your plan used to be to fly to France, then travel by road to Germany, stay there, and then fly home, it would be an entirely reasonable adjustment to travel from France, through Belgium and the Netherlands, on to Germany. Two countries not on the initial itinerary, yet you merely shifted your road route a little bit towards the coast. You might also find a better flight which takes you into Amsterdam rather than Paris, and substitute the Netherlands for France. Also a minor adjustment.
For certain changes after the visa was issued, there is a low probability of being caught, but the chance exists. There are places in the EU where you can walk across international borders half a dozen times in a single afternoon, and the itinerary would not list every single crossing.