Keep in mind that when you swithc, your "virtually" paying en entry cost that is time. Indeed you'll lose the time to build a new reputaiton in your new firm. Once this is done your carreer can be accelerated again.
This being said, if you feel you need to switch for your own perspectives (alignement with firm objectives, practices, promotion potential) :
- make sure you project yourself in the new company for min. 2-3 years, otherwise it doesn't make sense
- lay out clearly your motivation, to explain this in the future interview. Could bed either a specific practice you're interested in and is not in your previous firm, or some developpement potential within the firm that are not the same
Hope this helps