The diphthongs [ei], [ou] generally became [ī], [ū] in Quenya, but as a somewhat earlier sound change, the combinations yei, wou became yē, wō. The clearest description of this change appears in the Outline of Phonetic Development (OP1) from the 1940s:

The stressed sequences yei, wou were the first to change. In these cases, loss of ı̯, u̯ by dissimilation producing yē, wō appeared already in AQ (OP1: PE19/54).

These sound changes are also reaffirmed in later notes. In a chart of diphthongal developments from the late 1960s, Tolkien notes that usually ei, ou > ī, ū but became “ē after i̯, y; ō after u̯, w” (VT48/7), but in these later notes there was no indication that stress was required for the sound change. There are also hints of this change in the developments for the suffix -itë, one form of which was “éte (from eiti after i, y)” (PE22/155). This last text also provides the clearest example of this sound change:

• √SKEY [> skeyeite > skeyēte] > Q. xiéte “passing, impermanent” (PE22/155) [with eye > ie as usual].

Conceptual Development: See the entry on how [ei], [ou] generally became [ī], [ū] for details on the conceptual development of the diphthongal pair ei, ou in Quenya.