This is the biggest problem going forward. I wrote about the problem many times on my blog, in talks, and as premises in my sci-fi novels Sitting in your cubical with your perfect set of test suites, code verification rules, SOP's and code reviews you wont want to hear this, but other companies will be gunning for your market; writing almost identical software to yours in the future from a series of prompts that gene…
How do you get an extensive test suite?
For me with any new release of my winery production software I re-ran every job put into my clients production systems from job #1, about 200,000 jobs; Before going into production we checked all the balances, product compositions and inventory from this new software matches what the old system currently says. Takes about an hour to re-run fifteen years production and even a milliliter, or milligram difference was enough to trigger a stop/check. We had an extensive set of input data I could also feed in, to ensure mistakes were caught there too.
I expect other people do it there own way, but as a business this would be the low bar of testing I would expect to be done.