Changes in Database Design

I got my website designed from a designer and now I’m doing my studies using the code.
There is a product table where product id is an integer of length 4 which I changed to 6 as the number of products exceeded the range. I also made changes accordingly to the cart tables where the order item data is held. There was an error after the changes that the cart page shown during confirmation was not showing up now. When I asked my designer about this, he said databases can’t be changed as they contain primary keys and references. I understand that key and references need to be taken care off but I don’t understand why a databse can’t be changed. :o
I’m a naive, please help as to how make changes to database so as the script runs fine. I don’t know what he did to databse then, as now it is running absolutely fine.

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