For semi flexi you can generally perform withdrawals of any extra payment but there’s a small fee associated with it and you usually have to go to your bank to do it. I’m doing kinda the same thing as you, I moved some FD funds to my mortgage account because FD at 3.8% is less money than the mortgage 4.x %, so I’m “losing” money by incurring more interest over time in the mortgage account if I kept my FD.