Trusting my developer


I have a purchased a PHP script that needs to be updated but I need to keep the customisations I have made. I am not a techie person and would like to know the best way I can protect my website being copied by a programmer as I have paid for the scripts licence.

Below is what the developer said to me:
" We will copy the complete source code of website and then fetch the updates from the link of script you will provide us. **
After that we would compare the codes and make your changes available to your new updates. We have a test server with us and we will show you our work over there. Then you can confirm the update and customisation is done to your satisfaction

1). Can anyone suggest ways I can have my update done and keep my customisations?

2). I am not sure whether this way they will mean they will have a copy of my website and will be able to clone it for free from me. Can you tell me?

3). How do they compare the codes and still do the update accurately, it sounds really hard unless there is software they can use that makes no errors…?

Thank you in advance :relaxed: