Notice Period Buyout Confusion: Is Basic or Gross Salary the Valid Option Now?

I had joined 5 years ago, and at that time, the Notice Period Buyout clause was based on Basic Salary. A couple of years later, the company changed its policies and made it based on Gross Salary. I don't recall signing anything accepting this change. What will be considered valid now if I am trying to buy out the notice period?

Please provide your insights considering the following scenarios:

Option 1: I signed a new updated offer letter with the Gross Salary condition.
Option 2: I did not sign a new updated offer letter; the company simply sent an email about the change.

Option 1: Acceptance is explicitly given by you.

Option 2: You were made aware and by not objecting, have thereby given implied consent.