AAM DataSource Integration codes are solely used for the purpose of ID syncs. Unless you have other sections of the site or other properties (like mobile apps or other websites) using that test123 integration code, there's no harm in changing the integration code for that data source in the AAM UI.
In terms of why is it still working, it sounds like you're either in the process of migrating or just migrated. However you did the ID sync before (in a pure DIL implementation, you may have been using the data source ID or DPID), must have been going long enough that you have quite a few ID lookups stored in that data source. Those are probably still pretty fresh, so your onboarding is going to continue to work just fine for now. Give it 6-12 months and you'd probably find that your number of "Records with no matching AAM ID" in your onboarding report would increase. in other words, your existing lookup table that you've built up over time is still there, but you're probably not adding any new ID syncs to it. The trait to look at would be your synced audiences trait for the data source in question. I'm going to guess that your daily unique visitor count is trending downward because you're probably not seeing any new ID syncs. You're probably also seeing an error code in the AAM response to your Analytics or ID service calls stating that it doesn't recognize the ID or data source.
It is hard to really get into details on a community post like this. If you need more help, I'd recommend you open an AAM case and someone from our AAM support team can help answer more questions. If you happen to be an ultimate support customer, then we can probably even help you get in touch with the field services team where they can do a little desk-side coaching with you.