Change every year. FInally doing less this year and it was a smart move. Like others, I've gotten tired of doing so much change ... so I think a more sustainable approach is to stay with a design once you get something your happen with and just do minor enhancements and improve the sequencing. My sequencing has always suffered because I did too much change on the display elements side. On the flip side, it was the change that I think fueled my general excitement to do the show each year, so without anything new ... it can be a challenge to get motivated, etc ... unless you make the change in the sequencing, etc.
The general pattern for me has been to go from typical (house oriented design) to more of a Holdman cloned design ... to more of a show centric design where I use my yard (shared front yard between homes, since its a two home display) ... and also to simplify in many ways. The first year I had so much going on ... that is truly was overkill and doing too much (poorly) vs. doing less very well. This year I actually simplified and removed what amounted to about 8-9 16ch controllers of stuff ... and I don't miss it. I honestly think the less is more can really work if you think about what the key elements of your show are going to be. For me, that was getting away from the typical house and windows things ... and moving into display elements ... where the houses are just a couple key elements (roofs, wreaths) ... and the rest centers around megatrees, tree lines, rgb toppers (stars, crosses, etc.) and matrix(es), arches. I've been changing every year, even when I didn't want to. There just seems to be some change that happens, whether you plan on it or not. For example, my neighbor puts the wrong star topper on the megatree ... or stuff breaks and you have to change hardware or channel assignments on the fly ... so even when its the same, its still a little different. I have found xlights to be a godsend for allowing easy changes each year.