Sh56 will require changing the pedals.
I lost confidence a while back with Sh51 cleats after several close calls. I got some Sh56 and used them for a while and, over time, increased the tension on the pedals to about mid point. I then changed back to Sh51. At the same time I reduced pedal tension to minimum.
I now use Sh51 on my hybrid and CX bikes and Look Keo on my road bike. Wouldn't be without kleats now.