This isn’t something that can be done using PHP. When a repeater field is generated there is a ‘clone’ version of the row that is hidden. When you add a row ACF copies that hidden row to make the new row.
There is nothing in ACF that will let you conditionally change the label of a field, or any other aspect of a field, based on another field. I’m not the developer, but I honestly don’t see features like this making it into ACF.
The only way to accomplish what you want would be to add custom JavaScript. If you’re interested I can give you some ideas on how to accomplish this, but you said in the op that custom js was not an option.
