I tend to see three methods;

1) Disk fails, little warning and auto rebuild from parity.
2) Disk failing, sometimes get warnings and auto moves data elsewhere.
3) Disk not happy, maybe a few warnings or not available for allocations but requires manual servicing to start the data rebuild/move process before the engineer arrives.

Support typically are aware if the disk is ready for replacement but not sure what info from the SP uploads they check for that, I suspect the estimates they sometime give are generic based on disk type/size.

The fun tends to begin when the extra load from the rebuild fails another disk and/or when inserting the new disk doesn't go to plan. Three different service companies and over a dozen different engineers in 5 years has led to random events during replacements but no data loss.