I left the E5 boom at the original length of 640mm. The stock TT length was 632.5mm. I shortened it by 2.5mm down to 630mm. I red Loctited the TT bearings at 190mm from one end and 205mm from the other end of the TT. I left the boom support rods at the stock length of 480mm.
I guess your real question is whether to leave the boom, TT and boom supports rods at the longer length you have now when dropping the blade size to 556mm. Less hassle an gives you the ability to convert back to accommodate the longer 600+ size blades. It should fly pretty well with an unnecessarily long tail, but you may accentuate the pendulum effect. Best to keep the boom long enough to avoid a blade strike, but not much longer than that.