I recently upgraded from a 29G to a 75G. I was fortunate that I could keep both tanks running for a couple of weeks while I slowly moved fish over to the new tank. If you have to take down the 55 to install the 75, I would suggest getting as big of a container as possible (like a big storage bin), fill it with tank water, put your sponge filter in it (and a heater too if you have one), and place all the fish in there. If you have extra wood/decorations, they can go in the container to provide some shelter for stressed fish. You can also store your plants in that container with the fish until you are ready for them in your 75G.
I think the key is to block off enough time to get the new tank set up and scaped ina single day. This way your fish don't spend too much time in the container.
Once the fish are in the new tank, just do daily water changes until you have no ammonia or nitrite.