Take the 55 down and put things in rubbermaid tubs thru the room/house.
your wife will love you for this. Remove the 55 and in comes the 125. Put the new southdown sand in first then scoop in your old sand on top. You might see a slight ammonia rise due to the disturbance of the sand bed but the existing batera should take quick work of this.
Then add your LR
back. Remember to have your LR
covered during this process in the tubs or buckets.
Once the LR
is in place start t ofill with water. Use the 55 gal
from the old tank first and then use new water for the balance.
Should take a day like you said and the tank should be ready for additional critters within 2 days.