I think if they lost ALL the Superman rights and all the characters,they would have written him out via a crisis style story,give ACTION COMICS to Batman too then move on.
However,DC owns most of the larger Superman lore lock stock and barrel. The only pieces they could conceivably lose is the stuff in ACTION #1,Superman as a book title,Clark Kent as the secret ID who works at a newspaper,Lois Lane ,the original Superman look...and that's it. As I understand it,DC could still have a character called Superman star in Comics because they own the trademark,the Logo etc. Think Captain Marvel. Until the New 52 they used the Captain Marvel name in stories but not on covers. The book though probably couldn't use the Superman name on the cover (perhaps substitute a nick name they own The Man Of Steel), change the suit where it wasn't the Shuster strongman trunks and tights model,drop Clark Kent as a secret identity and either have Lois die,dissapear or retconned out.Everything else would remain unaffected.
Note though that in much of the 2000's and the 2010's,Clark Kent has been MIA much of the time, there have been times we've barely seen Lois, there were long stretches where he hasn't worn the classic suit and even times where he was replaced in his own books for years at a time by variant characters (New Krypton anyone?). I don't think that is a coincidence. Either trial balloons testing plan B approaches if they ever lose the Action #1 copyrighted stuff,or to avoid paying the heirs back royalties.
Also,the heirs could conceivably use their copyrighted material had they won the suit,but they could only use those parts they owned. But no Jimmy Olsen or Perry White. No Lex Luthor. No Metropolis or Supergirl or Kryptonite or Krypto or Fortress of Solitude or....well anything else Superman related.
In other words even if DC /Warner's lost,they still would have won ,as they own nearly most of the iconography of the character.