What kind of corals? How deep is your sand bed? And how old is the 55g? Is the 125g going where the other is, or can you keep both running?
Best case senario: Get the 125 running, and some of the new bagged live sand and let it run a couple of days and slowly add the rocks, fish. As doing this over the course of a couple of weeks, add a few cup fulls of old sand daily.....the good thing is your upgrading by double, your 125g will absorb any flucuations easier than if say going to a 75g.......Adding the old sand is the catchy part.....don't buy any new rock for the tank and when you do, add it slowly as well.
If its going to the same spot, drain as much water and try to slide it by sitting on the floor and pulling it by the base.....moved my 37 tall that way, nearly full of water.
1 to the other in a day............move the rock and fish and corals into tubs.....leave out the sand, you can live without it.......take out the rock and coral, drain the water a save all of it for the new tank
set up the new tank, put all the water from the old into the new...place the rocks and fish in, and add new water......by putting the old water, then livestock, then new water.....you will be acclimating them better than just mixing all the water and then putting them in..........because you are doubling the volume, and won't be the same chemistry....like not acclimating from the store and just dumping them in your tank.....
Obviously, its more risky the less options you give yourself and just go from one to the other in a day....but if thats your only choice..........
After all is settled after a few weeks, begin to put your Live Sand in SLOWLY over the course of a week or.....hopefully your are replacing heaters and pumps, so you can keep the sand in a bucket for a few weeks with a pump and heater.......top it off with some dead stuff, live stuff and call er good.......after a month I would start adding more rock slowly and not add to the bioload until all your rock is in.........
If you don't have all SPS that is super finicky should be alright....but leaving out the sand and adding later SLOWLY so the establishing bio filter can absorb and process it easier and slower, I think is your best bet.
But then again, Murphy usually visits around these times....Good Luck, plan it out before and HAVE FUN!!!! I envy more room youll have.
I probably left out something because I am tired, but someone Im sure will point it out. SLOWER the better, thats the key to the transfer.