I upgraded a 26g bow front to a 40b years ago. The 40b had to go where the 26 was sitting which meant a full tear down, move out, move in and set up new tank. I bought a 20g tank to house the fish, drained water from the 26 to the temp tank and also moved the existing filter to that temp tank to keep it running.
After cleaning out the 26g, moving it out and the 40b into place, I set up the 40b completely new - new substrate, new decor, new filter, all new (treated) water and only brought over one large annubias from the original tank. When I moved the fish back, I also brought over the original filter to run along side the new, larger filter for several months. Voila!...all fish survived.
But it was stressful as I had only ever had the one tank for a few years and was still pretty new to the hobby. But with the help of my 20-something yr old (at the time) son I was pretty proud of us. While I sold the bow front, of course that 20g temp tank became #2 permanent tank and it spiraled just a bit from there. Today, I'm at 5, down from a high of 7 and looking to get down to 2, maybe 3.