The case focuses on whether the song is copyrighted, and if not, whether Warner/Chappell has to pay back the licensing fees it has collected.

At least for now, a federal judge is allowing a lawsuit over the song "Happy Birthday" to assert claims on behalf of those who have paid licensing fees dating back to 1949.

In September, plaintiffs looking to free the song from copyright control scored a major victory when U.S. District Judge George H. King ruled that Warner/Chappell Music did not own a valid copyright in the Happy Birthday lyrics.

The plaintiffs then sought to amend their complaint a fifth time to cover the "millions of dollars of unlawful licensing fees" collected over the decades. To survive statute of limitations, the plaintiffs are also aiming to detail the concealment of truth regarding the "Happy Birthday" copyright licensing regime.

On Monday, the judge gave a nod, though this hardly means that Warner/Chappell is definitely staring at the possibility of disgorging profits on a song estimated to rake in $2 million a year.

"While there are questions about the ultimate viability of the added allegations in the proposed Fifth Amended Complaint, it is not optimal for us to decide these questions on a motion for leave to amend under the rubric of futility," stated the judge in an order. "The issues raised by Defendants regarding whether Plaintiffs have adequately pled delayed discovery or fraudulent concealment are better resolved on a motion to dismiss."

Warner/Chappell is still pushing the judge to either reconsider his September ruling or authorize an immediate appeal.

Meanwhile, a charity identifying itself as being co-founded by Patty Hill, the schoolteacher who wrote "Happy Birthday" in the late 19th century, has asserted its own dog in the fight. In a motion to intervene filed last month, the Association for Childhood Education said there was a "very real likelihood" that it was the true owner of the copyright to the lyrics.

On Monday, the judge allowed the charity to intervene.