I like the idea of changing jobs but I do think instanced content should lock you into the job you entered with until you leave the dungeon/instance.

However, unless the content was designed intelligently, people would roll with whatever was used to clear the dungeon first, which in essence destroys the unique idea of the concept. Conversely if these dungeons were designed intelligently without firm jobs in mind much like WoW's early dungeons. People could roll in with whatever they have and take a go at it which in my opinion would be fantastic.