I just moved a 125 gallon from one house to another. I didn't go to a bigger size, but I was concerned about disturbing the sand bed and killing my bacteria. I did it about the same way that the poster before me did...I bought several rubbermaid tubs and placed the sand in it (I would have used large rubbermaid trash cans, but I was moving so I needed to be able to transport the weight easier than large trash cans would allow) Then I filled them with as much original tank water as I could and still be able to move them (if you are just upgrading size, you wouldn't really need to worry about being able to move them) I placed half of my live rock and inverts into one rubbermaid tub, and placed a powerhead in there for circulation, and I placed my fish and the rest of my live rock in an insulated cooler, and placed another powerhead in there for circulation. ( I wanted the cooler to maintain the temperature of the fish's water because the day we moved was a hot day. ) I tried to hang my Emperor power filter on the side of the cooler, and it fit, but the stupid thing didn't have enough ooomph to pull the water up that far, so I scrapped that idea. The advantage you have is that since you aren't changing locations, just tank sizes, you can save most of your original water.
Then we moved the tank, I scooped the sand back into the tank, filled it with the original water that I had saved, mixed new water to replace what I couldn't bring with me, and started the pumps and filters. The water was really cloudy but I had to put the inverts back in because the water temp was dropping fast in the non-insulated rubbermaid tub. Everyone did just fine. The next morning I checked everything and acclimated the fish who had stayed in the cooler overnight, and placed them back in when that was done. All of it went over flawlessly, aside from being a major PIA.
Also, just to play it safe, I purchased two bags of the Bio-Spira bacteria made by Marineland and added that. I'm not sure if it helped at all, but I figured it couldn't hurt. I'm sure that disturbing the sand bed affected the bacteria levels in a negative way. You may want to consider buying some.
I'm sure that increasing the size of your tank is a similar process, except that you have to get the live sand to make up the difference between the two tank sizes.