Remortgage
Remortgaging is the process you go through when you change your mortgage lender without moving property. It is fair to say remortgaging is common practice in today's mortgage market and approximately 50% of mortgages completed annually are remortgages.
There are various reasons for borrowers remortgaging however the main reason is they are looking for a better deal than they are currently getting from their existing lender.
Lenders typically reserve their best products for new customers and only offer their existing customers a small subset of their full product range. Although they may offer you another mortgage product it may not be as competitive as you could get from another lender.