Whenever EIGRP detects a topology change it first records the change in the topology table, then updates the RD and CD of the neighbor that advertised the changed route.
EIGRP then identifies the one route that provides the least CD taking into account the updated CDs, FC is not yet invoked.
Only after identifying the neighbor with the least CD, EIGRP verifies if the neighbor meets the FC and is therefore a Feasible Successor. If it is EIGRP will promote it to the Successor and start using it immediately.
If the neighbor does not meet the FC, EIGRP will go Active on that route and begin sending out Queries to find an alternate best loop free path to the destination.