Do not change the encounter. I for one would be exceptionally pissed off if I found out my DM had changed an encounter without a good darn in-game reason, only having done it because now, aww gee, the character's now have an in-character idea of what the encounter is. Just why in the world would there be more creatures there? What kind of explanation would there be for it?
And then once you have your explanation...as I'm sure you'll be able to find one...would you have used it if the characters had no foreknowledge or planning of what the encounter would be were they to, for some reason, go back to it a second time around? Or is it only because now they have perfectly legit, in-game intelligence on the situation?
Had they actually meta-gamed, I might change the encounter around a bit. But I don't believe saying it was a tough encounter OOC really gave them much to go on. They did some scouting, learned some info, all in-game, and came to a perfectly reasonable in-game conclusion over the matter. Unless you intended on changing the encounter before the PC's came to the conclusion that they were going to show the shadow dragon what for, don't alter the encounter. That's just bad DM'ing in my book.
Being that I primarily DM tabletop, most of my playing experience comes through LARPs, but the logic still applies. Whenever I've found out about an encounter that was changed at the last minute because high-powered PC's decided to do it, I take a decidedly dim view on the individuals who decided to do that. Particularly since the reverse rarely happens; i.e., low-powered PC's inadvertantly run across something too hard to handle, and fight because they're still new to the game and don't necessarily know any better.
For my part, I don't change an encounter that's too hard for the PC's to handle, either. What I do do, however, is change the monster's tactics. Is the fight too tough? Well, then once the PC's wound the creature, perhaps it flees, not having suspected the PC's could harm it in the first place. Too easy? Then I mercilessly tear into them with the best tactics I believe the creature or creatures can muster, taking every advantage the monster could conceivably think of, and using it to beat the PC's to within an inch of their lives.
And that's my suggestion to you. Do not change the encounter. That, I repeat, is just bad DM'ing in my viewpoint, to punish the characters for being intelligent and making perfectly reasonable in-game conclusions. Do, however, feel free to squeeze every last drop of strategical know-how out of the shadow dragon and any of its minions. When negative energy attacks don't work, claw them up into little bitty shreds. When melee doesn't seem to be the way to go, have it use whatever spells it has available from a distance. If its minions are locked up behind a wall of force, have it retreat to somewhere those minions can get at the PC's if they follow. But don't - do not - throw in more monsters or change the shadow dragon or whatever. Give them the encounter they would have faced if they hadn't made any plans.
Because I know my player's grow a bit frustrated when their plans fall apart on them without my having cheated them on it. Which changing the encounter on them would equate to - cheating. Meta-gaming. Changing the shadow dragon to something else only because the PC's have planned on fighting a shadow dragon. It's one thing for the PC's to be wrong to begin with, such as in my Ravenloft game, where they believe the weretiger wizard they faced was instead a rakshasa, but another thing entirely for them to be right and then punished for being right (when no meta-gaming was involved, or a very, very minimal amount).
