Notice Period Buyout clause was on Basic Salary changed to gross salary by company

I had joined 5 years back and then the Notice Period Buyout clause was on Basic Salary.
Couple of years later, company changed policies and made it on Gross Salary.
I don't recollect signing anything accepting the same but what will be valid now when I trying to Buy out the notice period?
Please provide your inputs Assuming below scenarios:
Option 1: I did sign new updated offer letter, with Gross Salary condition.
Option 2: I did not sign new updated offer letter. Company just sent an email about the change.

Option 1
Acceptance explicitly given by you .
Option 2
You were mafe aware and by not objecting have thereby given implied consent