Late in Tarquesta [TQ] the initial combinations ps-, ks- came to be pronounced ups-, iks-, a sound change described in both the Outline of Phonetic Development (OP1) from the 1930s and the Outline of Phonology (OP2) from the 1950s:

In TQ though the spelling is retained ps, ks initial are commonly rendered by simple s or ps, ks > ups, iks (OP1: PE19/36).

In TQ ups, iks, rare in PQ, became frequent as colloquial renderings of ps-, ks-, unless a closely connected word ending a vowel preceded (OP2: PE19/79).

In the 1930s, these initial clusters sometimes reduced to s-, a phonetic development also mentioned in descriptions of the Feanorian Alphabet from the 1940s, emphasizing this sound change was not reflected in spelling:

The TQ colloquial pronunciation of initial ps, ts, ks as s is never “correctly” represented as 8 (though this occasionally appears) (PE22/45).

There are no obvious examples of these sound changes in Tolkien’s writing, since they are hard to distinguish from the syllabification of initial ṣ followed by metathesis: ṣp-, ṣk- > usp-, isk- > ups-, iks- (which probably helped produce these late TQ pronunciations of ps-, ks-).

Neo-Quenya: Most Neo-Quenya authors ignore this sound change and retain the spellings ps-, ks- (or x-). The same is true for the entries in Eldamo, where the original spellings are retained. It’s probably best to consider ups-, iks- as optional changes to pronunciation only.