To change the original terms and conditions of your mortgage you will need to apply for what is known in banking as a 'novation'. This means drawing up a new agreement with the Bank and then generating a new public document, i.e. a new mortgage deed and a new entry in the property registry.
As for the reasons for applying for a change in your mortgage terms and conditions, the most common scenarios are as follows:
You need to finance new projects
This tends to be the reason for applying for an extension of the mortgage amount because the terms and conditions of a secured loan are usually more attractive that those of a personal loan.
Your personal and risk situation has changed
For example, a change or improvement in your employment situation could justify the renegotiation of the spread applied to your mortgage.