To be sure, not all the justices agreed with this additional direction. "Why the Court embarks on this peculiar path is beyond me," said Justice Scalia, with Justice Thomas concurring, said, as they dissented from the second part of the decision.

Although it's difficult to predict what will follow, the Supreme Court's actions provide several lessons for plan sponsors. On the positive side, the decision affirms the fact that the plan document is the plan document, Komornicka says.

At the same time, the majority of the justices seemed to go ahead and give the district courts the ability to reform the plans, Elizabeth Vaughan says. Observers will have to wait to see how the courts apply equitable relief to the case.

To minimize the potential for challenges, companies -- either through their boards or an executive appointed by the board -- should formally adopt the written plan document, as well as any amendments, Shpiece says. "If the company cannot prove that a document or amendment was formally and properly adopted, it will be very difficult to enforce a provision in it."