Warranty? Or Service Maintenance Agreement? Either way what's covered should be in your documentation.
If it's out of pocket make an investment in one of these first, to see if the fluid actually needs changing:
https://www.amazon.com/CREACLE-Diagn...-2-spons&psc=1
The BMW interval is mileage based, not condition, so it doesn't hurt to make sure the condition warrants it.