Composite doors come with lots of different types of locks. This particular one was a key/key lock which means you have to use a key to open it from both inside the property and outside.
The owners wanted to change this to only needing a key when outside, and having a thumb turn on the inside. This is safer in the case of a fire, as you know you don’t need to find the key to get out.
The process involved in changing these types over involves both a change of the locking mechanism and the handles. As an experienced Tewkesbury locksmith, I carry lots of spare locks and handles on my van, but I can also order in anything you particularly require, should I not already have it.