Your players always pick the path that extends the plot, whether or not they know it. You need them to take a left fork at the road and they picked the right? Amazing, it has same ambushes and character events on the road just with a different town at the end.

Until a map is drawn there is no geography, so don't get caught up in it. Again, they need to be in Mashtown and instead went to Dingleshire? Thank goodness the mayor of Dingleshire knows an old passage that leads right to Mashtown, goes right through that mountain I just made up. This also totally let's you buy time for plot growth.

You can switch stat blocks if you want, or just shift names and types. That zombie giant is now a giant scarecrow. It's a construct instead of undead and now deals fire damage instead of necrotic. It still has undead resilience but now it's pumpkin resilience. It's also like 50% more memorable and that guy who memorized the Monster Manual can't be meta about it.