help

Our house grantor and grantee is my ex-husband and I . and both of us name on the mortgage. But our house deed just have my ex name. Recently, I want to early payoff our loan, before I do it, I need to remove my ex name from our house deed and put my name . My mortgage company told me that I need to fill out the assumption form to remove my ex name from our loan and then I can remove my ex name from our deed., My issue is why do I need to fill out the assumption form to remove my ex name from our loan ? I don not want to borrow the money from company again . I want to payoff it .My ex has agreed to keep his name on our loan and just remove his name from our deed and after his name remove from our deed I can payoff our remaining balance. I wonder do I have to fill out the assumption form before remove my ex name from our house deed?