Old worlds should be compatible with newer versions, so you should be able to just load it and it should be fine. Some issues could still arise, some recipes have been changed in newer versions, so if you have a world with a crafter you placed in the first version, when loading it into a newer version, the recipes stay the same until it is destroyed and placed again. There could be other problems that come from loading old worlds into new versions, but I have tried my best to preserve compatibility.
About the rendering, as far as I've tested, it does only 4 texture swaps every frame (6 if goddess is on screen), where before it did between 2 and 4 for every structure (depending on the structure). Texture swaps are slow, and the performance impact of 40 texture swaps every frame is a lot more than drawing 40 sprites. The engine only renders stuff on screen, so it doesn't swap textures for off screen stuff. So in old versions it performs worse when zoomed out because it needs to swap a lot more, not only because it needs to draw more sprites.
You are right that when zooming out, it should be slower, and in old versions it is a lot slower because of texture swaps, but after v9.8 it performs about the same on all zoom levels, because the main difference in performance between zooms is no longer there. Drawing more sprites itself has almost no impact on performance in comparison to other things the game does, so you won't notice it between zoom levels.