greymarch said:
Should DMs change the stats of official NPCs?
Yes- if it fits with your ideas for your campaign. I mean, it is your campaign afterall, and if you want an NPC (even an official one) to be a certain level, have certain abilities, etc. then by all means change it.
For example, the Cat Lord from the ELH is a 37th-level rogue. Suppose you think he should only be 20th-level. Then make the changes to reflect it. Is it bad? No. Is it wrong? No. Should everyone use your new and improved Cat Lord? Only if it fits into their idea of the Cat Lord and fits their campaign. Will some people complain? Yes. They always do. Does that mean you are wrong for changing it? No, of course not. And anybody that says otherwise needs to go back and read the books.
Its your game, you change it as you see fit. WotC gives you the tools and materials. You build your game from those tools and materials changing, adding, subtracting, as you see fit. Whatever works best for your game is how you should do it.
Can it ruin continuity of the game, and the trust of the players?
The only continuity would be the continuity you instill in your game. Can it ruin it? Yes, if you let it. Ruin the trust of the players? No. If anything it keeps them on their toes.
Suppose you have a player that knows Elminster like the back of his hand. Well, you change your version of Elminster and give him a few levels of assassin. The player screams and complains "Thats not how he is in the ELH!"
Guess what? So what. Its your game. If you see Elminster having a few levels of assassin, give them to him. Remind the players, that it is your game, and in your game, that is how it works. There is no right way or wrong way to play D&D. There is only the "fun" way.