Remember, the linear slot diffusers aren't just a drop-in replacement - you'll need surgery on the wall openings and replacement boots connected to the ductwork. These boots are unlikely to work will wall mounted ductwork and may or may not work with ceiling/floor mounted ductwork.

You are best to keep the style you have. Different systems, and different age systems, all have different type of grills. The pictures you posted, shows that your grills are in nice condition, and look just fine.