There should be an option to automatically save the last two or three designed states or be able to ctrl z back to them. Could be saved as separate files, or just maintain the data internally. Preferrably would keep the data internal. I hate when I flip a switch, realize it was a mistake and have to redesign... Can take forever.
Design now is just deleted for members that have parameters changed. This prevents the need to design the whole model again. If you want to just design the members that were changed then please use Update design tool.