Willis plans to offer Towers Watson shareholders a £117.75m ($179m) sweetener to head off legal challenges targeting the merger of the two consultants.
Towers Watson shareholders may also receive 50.1% of an estimated $4.7bn of additional value that the deal is expected to generate.
Under the amended agreement, Towers Watson shareholders will receive a one-time cash dividend of £6.58 ($10) per Towers Watson share pre-closing, an increase of £3.37 ($5.13) per share over the original terms of the agreement.
The pair agreed an £11.4bn ($18bn) merger in June but four cases have been brought by Towers Watson shareholders in the US claiming the deal undervalues the business.
The revised transaction terms have been unanimously approved by each company's board of directors.
Both companies will hold shareholder meetings by December 16 to vote on the amended plans.
There will be no change to the number of Willis shares that Towers Watson shareholders will receive for each Towers Watson share, to the terms of the proposed reverse stock split, or to the management and governance arrangements originally announced.
As before, the combined company will be named Willis Towers Watson.
In a statement Willis announced that pending the approval of the combined board, in the six to twelve months following the completion of the deal, it intended to begin returning capital to shareholders to achieve a leverage ratio for the merged company broadly in-line with Willis' investment grade rating profile.
Willis CEO Dominic Casserley said: "We believe that the combination of Willis and Towers Watson will create significant value for our shareholders, and that together we can achieve significantly more at a faster pace than we can independently.
"In order to enable Towers Watson shareholder support, we are therefore agreeing to allow Towers Watson to increase the pre-close cash dividend.
"This is not a decision that we take lightly. However, for an increment of $179m (50.1% of $357.4m), Willis shareholders have the opportunity to receive 50.1% of an estimated $4.7bn of additional value that the deal is expected to generate."
In a separate statement, Towers Watson CEO John Haley added: "We are pleased to announce the revised terms of our proposed merger with Willis, which are based on our extensive engagement with stockholders and negotiation with Willis as well as our commitment to completing this compelling transaction."
The consultant has also received regulatory approval for its takeover of PMI Health Group, which prompted a rebrand.
