Pyrex, the term, was originally a brand for borosilicate glass. It has become partially generic over the years, like Aspirin or Kleenex.
Aspirin is still a trademark of Bayer in Canada; if they want to switch to ibuprofen instead of aspirin as the active ingredient, and sell it as Aspirin, they could. Similarly, if Kimberley-Clark decides to start selling sandpaper and call them Kleenex, they could.
If a trademark becomes fully generic it can be ruled so by a court, at which point the trademark protection is lost. Aspirin is like that in the US. But that is pretty rare these days.
But if someone sold ibuprofen as "aspirin", doctors would rightfully decry the move as likely to get someone killed. It's a generic but it has a specific meaning.
Selling soda-lime glass as "pyrex" has gotten plenty of people injured, but evidently none severely enough to sue Corning's pants off.
Aspirin is different - the US trademark for the name was explicitly expropriated from Bayer by the US government during WW1 and became a generic... that didn't happen to Kleenex or Pyrex.
Aspirin is still a trademark of Bayer in Canada; if they want to switch to ibuprofen instead of aspirin as the active ingredient, and sell it as Aspirin, they could. Similarly, if Kimberley-Clark decides to start selling sandpaper and call them Kleenex, they could.
If a trademark becomes fully generic it can be ruled so by a court, at which point the trademark protection is lost. Aspirin is like that in the US. But that is pretty rare these days.