Ooo~ sorry for late reply.
(For some reason i couldn't reply to your reply before?)
Well, Goofy's redesign was made only for the 'new' Mickey Mouse Shorts, but his "classic" design was still being used in every other non-Mickey Mouse Shorts Media, including future shows. So his case is the same as Flinheart Glomgold, Scrooge McDuck and every other not original character that got a redesign in the show; it was only made for a specific show/media.
And yeah, Della's "classic" design is still being used in the comics post-DuckTales 2017, but she's not like Glomgold or Goofy or Scrooge, she appears like once a year, in really short stories, meanwhile her DuckTales design appear pretty often in many episodes, she's also in some official games and appears in DuckTales comics