If the frame is same cross section where the hangers were and where you want them then moving them up is pretty easy, and a good idea.
That said if you have crossmembers at your spring hangers (I bet you do) then they need to move along with the spring hangers.
Some times crossmembers are all the same and moving the hangers is even easier, crossmembers are already waiting.
Some trucks the frame tapers between the rear forward and rear most spring hangers, them moving them is problematic and cutting the frame and shortening in a straight section is the way to go.
