A gradual transition as recommended by others is definitely the way to go.
Might try setting up more roleplay encounters in the dungeon, where the (best) solution involves more than hack and slash or a spell. If you bring it to them in a place where things have not been fastforwarded, it will be easier to introduce it in the place that has. When the kobolds and goblins are fighting, who do they side with? Classic Sunless Citadel. In epic encounters this might involve a city of djinn or salamanders with many character levels themselves, where the place one needs to stick his sword is not immediately apparent.
Another good spot for advice... http://archive.wizards.com/default.asp?x=dnd/ab/20060824aLink
"For each roleplaying encounter, you need to prepare three things ahead of time: what the PCs can gain from the encounter, what the NPC might accept in exchange, and what skills or class abilities will help them in the encounter."
Types of RP encounters: "a passage encounter, a resource encounter, an information encounter, a talk-or-fight encounter, or something else."
"One of the best ways to make a combat more entertaining is to set it up as a roleplaying encounter first. The knight who brags he can spit the party members like piglets on his lance, the giant who toys with his food, or the evil wizard who pretends to be their friend before betraying them one night during the midnight watch are all potentially more interesting than merely rolling dice. If the party is going to meet one of these, prepare some dialogue ahead of time -- and set the initial attitude to Hostile."
Good luck :)