When I read that yesterday was Cellophane Tape Day, my first thought was “Huh? Is that a newly invented alternative to scotch tape?” No, it’s just the official generic name for scotch tape.
When the trademark becomes generic (scotch tape, kleenex, hamburger), reverting to the untrademarked term (cellophane tape, facial tissue, bun sandwich) is a process similar to a retronym, like Film Camera or Landline Phone. I don’t think there’s a proper word for this type of reversion.
Later: Turns out that today is National Bun Sandwich Day AND National Facial Tissue Day! I chose my examples amazingly well!
Polistra's Mill 