"How often do they usually do significant design changes?"
Don't worry, there have been no significant design changes since the 1920's. Maybe a few improvements due to new materials.
Seriously, with the exception of internally geared hubs, and maybe tires I really do not see anything that really has improved much since the late 80's. The stuff just weighs a bit less.
2000 Montague CX, I do not recommend it, but still ride it.
Strida 3, I recommend it for rides < 10mi wo steep hills.
2006 Rowbike 720 Sport, I recommend it as an exercise bike.
1996 Birdy, Recommend.
Wieleder CARiBIKE (folding), decent frame.