Does the Redmi Buds 6 support single earbud replacement or reconfiguration of the left and right earbuds?

Yes, it supports this feature. You can use either the left or right earbud independently, as the earbuds can be reconfigured for mono audio use. This means you can replace a lost or damaged earbud and still use the remaining one, with the ability to pair it to either side depending on your preference.
For your reference, the steps are as follows:
1. After replacing a single earbud, place both earbuds in the charging case and close the lid;
2. Open the lid, and while keeping the earbuds in the case, press and hold the function button on the charging case for 20 seconds. The earbuds will automatically reconfigure and pair with each other.