CFOs hate second-lien loans. They're really expensive—about 2 percentage points more, give or take, in annual interest-rate costs than traditional first-lien debt.

Which explains companies' sudden rush to take advantage of the booming leveraged-loan market and issue new first-lien debt—at falling yields, no less—to help pay down the pricier obligations.

Among those reaping the savings are 1-800 Contacts Inc. and human resources software company UKG Inc. Both are replacing lower-ranked loans (which get paid later in the event of bankruptcy) with higher-priority ones, saving themselves tens of millions of dollars in interest over the life of the debt. More than a dozen companies did something similar last month, and market watchers say it's only getting started as additional firms look to swap out expensive loans coming due in the next couple years.