I think this will encourage players to level up their alternative classes, looking at some of the new contents, plus the change in old contents, I'd say SE is trying to pump blood in lower level contents anyways, this way not only will the different tier contents be more active, they'll be more fun as well.

Obviously there will be conditions. For example, class and item level will need to be content par or higher (synced), role will need to be the same, if you entered the content as a paladin, you can switch to a warrior or a dark knight, but not a monk or etc.

The dungeons and raids are designed to be playable with pretty much any combination of classes anyways. I think this feature will make them more fun. And I'm sure players will find hilarious ways to play the current contents.