Think like this, you need to add the compulsory field of PO Number to your invoice table.
It wasn't compulsory before.
How do you not break everything?
It's impossible.
Or, even worse;
Projects can now have multiple categories. You change your code so all old requests still work, but suddenly half your systems give users the flexibility to add multiple categories and half don't.
Why's that worse? Because you suddenly have people creating projects in one system that only allows one category and then logging into another to add the extra categories because IT doesn't actually have to fix it. Total user nightmare.
And to top it all off all, and this is the actual killer, developers are having to use your never quite finished system which throws weird errors that people can't google like 'unable to add project', instead of 'violation of Foreign Key Constraint, FK_Project_User, on table Project' which tells them exactly the error.
Use standard DB access because there's a wealth of help out there, your inhouse system can't compare unless you're a big company.
Basically you've been asked to fix one of the fundamental problems of why enterprise programming is hard.