Yes, it is possible to use an HSBC mortgage for over 70s to pay off an existing mortgage. However, it is important to note that the terms and conditions of the loan may be different to those of a regular mortgage. For example, the interest rate on a HSBC mortgage for those over 70 may be slightly higher than on a regular mortgage due to age-related risks. Additionally, some lenders may require additional information or documents before approving a loan application. Therefore, it is advisable to speak with a financial advisor before taking out a loan in order to ensure that you understand all the terms and conditions associated with the loan.