Can I ask to revise CTC after accepting?

Hi Folks, I have 12 YOE, current pay in EY India is 23 fixed and 4 lpa is variable ( I got only 1.25 lakhs per year for past years), given interview recently in TCS, I was offered 28 as fixed and 4 in variable, in haste I agreed for it, and gave my consent in ctc break up email. Yet to get offer letter, now I realized I settled less for fixed, can I ask them to increase my salary now? What are the chances of outcome? Current position in EY: SC Band 4 TCS offered position: C3B