• Find a suitable container to house the fish while switching out tanks. A large trash can like you mentioned will work, but I personally prefer the Totes containers available at Walmart because you can more easily put your filter on them.
• Move some (not all) of the water from the 55g into the temporary container and top off with fresh dechlorinated water. If your temp container is elevated, it will be easier to drain afterwards (siphon).
• Move the filter and heater into the temporary container. Be careful with the heater if your temp container is plastic.
• Now then, don't worry about the fish, they'll be fine in the temp container for however long you need them to stay in there while you focus on the switch out of tanks. I've done this several times and the fish were in temp container for many (6-8) hours.
• Swirl the substrate around in the old tank water to get the detritus suspended in the water column and then siphon off the rest of the old, dirty water. The substrate doesn't have to be sparkling clean, nor do you want it to be. A good bit of the beneficial bacteria is harbored in the substrate.
• Now you can move the 55g and put the 75g in it place and begin moving the substrate over, along with any other hardscape (rocks, wood, etc.)
• Now begin filling with clean, dechlorinated water and top it off with what's in the temp container after you move the fish, filter(s) and heater back in.
Hope this helps...what kind of Cichlids are you keeping?