I actually like the move. In the longer term I think you probably upgrade your D and saving a roster spot and some cap space is certainly a consideration in deciding on such a move.
EJ may bang it soon but Larsson is the heir apparent for the Devils and Hedman is a typical slow developing big D who is just going to keep getting better - Whitney is never going to be a top flight D guy in fantasy to my mind and his health issues over the years scare me off.
I would do it.
Yahoo weekly starts 12 team league, keep 17 plus farm
GS, W, L, SV, GAA, SV%, SO G, A, P, +/-, PIM, SOG, GWG, PPP, SHP, HITS, FOW
3C: H. Sedin,Stepan, Stastny, R. Johansen, Hanzal, Anisimov,
3LW: Burrows, Vanek, Ott, Lupul, Dubinsky
3RW: Doan, Pavelski, Brown, Okposo,
6D: Franson, Doughty, Voynov, Subban, Markov, Gonchar, Karlsson
2G: Price, Emery
Farm: (< 100 games): S. Elliott, J. Bernier, Galchenyuk, Etem, Eakin, Atkinson