More precisely, Classic cards were just ‘mirrors’ of their respective counterparts from the Legacy set, which is still there unchanged — so that’s a formal excuse to not offer any refund, as per usual routines.
For reference: https://hearthstone.fandom.com/wiki/Classic_format .
What’s more, the recent changes having been made, you’re supposed to spend more, not the other way around (see, for instanse, this topic), and such a refund would have been inconsistent with this course of action.