For re-indexing repositories, we provide Index Existing in UI as below.
However, it is for indexing artifacts in a repo which was not indexed yet. Existing artifacts will not be re-index because Xray uses re-use mechanism. The idea behind this mechanism is to avoid the process of indexing the same artifact again and again.
Force ReIndex EST API is a new feature which fully re-index artifacts even if the artifacts are already stored in Xray.