Second the above.
As far as swapping the two, it can be tricky. The covertech knob uses an 8-32 cap screw to secure it. Typically, will VV, they will install the D-ring in a rather permanent way somewhere else on the hilt (pommel usually). Of course, you as the consumer could replace a covertech with a d-ring if you could find one that would work in the same way as the knob (being fastened to the hilt with a 8-32 cap screw). Unfortunately, items like that are difficult to find, the good looking ones anyway.
I had to machine my own. I've made two so far that are basically knobs that have a D-ring or Tri-Ring installed and are fastened to the hilt with the same 8-32 cap screw as the covertech. It looks good (one is even brass) however, the sort of funny thing about D-rings/Tri-rings is that they tend to smack against the hilt unless you have your hand over them which isn't always possible when spinning/dueling. Some folks, when making period specific sabers (such as OT or the EU now "Legends" era) have installed D-rings and mounted a tiny rare-earth magnet that would secure the D-ring when not being used to suspend the saber from the belt.