The Yap Islands (IOTA OC-012) is a group of four islands in the Caroline Islands. 
Until December 1980, these islands were part of Western Caroline Islands, now Palau.

From January 1981, the islands became part of the Eastern Caroline Islands, now Micronesia.

Yap State is one of four states in the Federated States of Micronesia, and comprises the Yap group of islands, together with 14 atolls east and south of Yap Island, as illustrated in the map of Yap State

Like Yap Island,some (if not all) of the islands that are now part of Yap State also used to be part of the Western Caroline Islands for DXCC purposes. 

For example, Ulithi Atoll, OC-078  was in Western Caroline Islands, according to the KC6CG QSL and the KC6AI licence (below).

The DXCC mapping is summarised below:

Palau22CurrentBefore 1981-01-01
Micronesia173CurrentFrom 1981-01-01

KC6AI licence for Falalop, Ulithi Atoll (tnx G3TXF and Richard Kohler):