Some of the world's biggest companies are readying their balance sheets for tax-law overhauls before a single bill has been introduced in Congress, trades that can net them millions of dollars in savings if tax rules are reworked.

Behemoths like Verizon Communications and Time Warner are buying back bonds they've issued, and selling new ones. These transactions can make sense even if tax laws don't get refashioned, but at least part of the motivation for two deals over the last five months was to get ahead of possible tax law changes, according to people with knowledge of the deals. 

More companies may look to do similar transactions in the coming months, one of the people said. Citigroup's financial strategy and solutions group, which advises issuers on corporate finance, told clients in a note last week that they should consider steps including buying back debt at prices above face value before the rules are changed.