Short question: How much would you adjust the pre-planned difficulty of an encounter to account for your players' preparations?
Slightly longer version with more context (mild spoilers for Lost Mines of Phandelver):
So my players got the ruins of Thundertree, met with the druid there, and were told he would guide them to the orc fortress if they could help him deal with Venomfang (small green dragon). Now, my party at the time consisted of only 3 players, all newbies, so despite my attempt to hint that the dragon was injured during the conversation (was planning on halving his hp and making him leave at a quarter hp), they walked, or rather ran after saying something rather insensible, convinced that they were absolutely no match for it. Now, about two weeks later in-game and several months outside, they return after going to Neverwinter and recruiting twice as many adventurers as their original number, including another permanent party member that joined us in the interim. Two weeks is a reasonable timeframe for the dragon to have healed significantly, but even at full health I doubt little Venomfang can so much as scratch ten adventurers working together (fortunately no wizards or sorcs, but they have a Cleric, Warlock and Bard), all level 3.
I've been toying with "while you were gone, the paranoid dragon enlisted some of the orc bands roaming around the countryside", but I'm not sure if it would be "fair" of me to increase the difficulty just because they thought of something clever.