welcome to the club. A quite common requirement.
Forget about chaning data in the case definition to have two different versions.
Of course you can set everything to execute the tests and then edit the cases to and execute those tests. The executed ones will keep the data from the execution, but that’s nasty.
If it would be just a questionof different environments, the configurations of TestRail yould help you. Good for reporting and easy creation with case mutation.
If you have simple cases (small number of steps if any), probably a small set of vary data you can handle it with manual copies. A custom field indicating the environment will help you to create the runs and finally for the reporting.
For a bigger number of cases having a lot of variants (e.g. more than 15) I implemented a tool for my customer using the the TestRail API.
Starting with a logical case containing the scenario in steps having parameters (my own syntax) and a custom text field wiht a table storing the data sets (multiple parameters for one case).
A separate template offers the field and makes these cases easy to identify.
The tool creates concrete cases per data set, having the parameters replaced with the data, ready to be executed.
Having a clear structure to store them, it is even possibleto update the cases.
Your requirements sounds like something in between…
If the data is only stored in one dedicated field, I see two other ways in addition:
A dirty way might be to store the data in a fresh result for a test before starting.
Create the runs and add a result storing the data for the test e.g. in the comments. Depending on the number possible better done via the API.
The silly way is add both data sets to the case but separate it somehow e.g. using a table. The tester gets instructed which one he has to use.
But going back to your first request: Why do you need to add the data to the test at all, if you’re using automation? Do your tests pick up the data from TestRail?
If not there is IMHO no need to keep them in the case.
If needed for any later reporting, the automation could probably add the used data to the result.
Well, I think at least some points to thing about…